Inkjet ink

ABSTRACT

There is provided an inkjet ink comprising a photo-acid generating agent which is capable of generating an acid as it is irradiated with light, a color component, and an acid-polymerizable compound which can be polymerized in the presence of an acid, wherein at least 40% of the acid-polymerizable solvent is a vinyl ether compound represented by the following general formula (1): 
 
R 13 —R 14 —(R 13 ) p    (1) 
 
     (Wherein R 13 (s) is a group selected from the group consisting of a vinyl ether group, a group having a vinyl ether skeleton, an alkoxy group, substituted hydroxyl group and hydroxyl group wherein at least one of R 13 (s) is vinyl ether group or a group having a vinyl ether skeleton, R 14  is a group comprising a substituted or unsubstituted cyclic skeleton and having a valence of (p+1), and p is a positive integer including zero).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. 2003-367804, filed Oct. 28, 2003;and No. 2004-252466, filed Aug. 31, 2004, the entire contents of both ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an inkjet ink.

2. Description of the Related Art

In a situation where a fairly large number of copies of printed matterare required such as in the cases of local advertisement bill,enterprise handout and a postern of large scale, a printing machineutilizing a form plate has been conventionally employed for meeting suchrequirements. In recent years however, an on-demand printer which iscapable of quickly coping with the diversification of needs and ofminimizing stocks has been increasingly utilized in place of theaforementioned conventional printing machine. As for such an on-demandprinter, an electrophotographic printer where a toner or a liquid toneris employed as well as an inkjet printer which is capable of achievinghigh-velocity and high-quality printing are expected to be useful.

As in the case of the printing machine utilizing a form plate, theon-demand printer employs a solvent type ink or a solvent type liquidtoner both containing a pigment and an organic solvent. This techniqueof employing a solvent type ink or a solvent type liquid toner ishowever accompanied with the problem that the organic solventvolatilizes to such an extent that cannot be disregarded when theprinting is repeated a certain number of times. Therefore, when thistechnique is employed, a problem of environmental contamination due tothis volatilized organic solvent will be raised, thus necessitating theprovision of an exhaust gas processing installation or a solventrecovery system.

On the other hand, in the case of the inkjet printer, since a solventtype ink can be handled in a closed system until the ink is dischargedonto the surface of printing matter, it is possible, through theprovision of suitable countermeasure for the exhaustion of solvent, toalleviate the aforementioned problem of environmental contamination.However, in different from the ink to be employed in the printingmachine utilizing a form plate, the ink to be employed in the inkjetprinter is required to have a suitable degree of fluidity for thedelivery of ink. Therefore, the ink to be employed in the inkjet printeris required to be sufficiently high in the concentration of solvent, sothat even with this technique, it is essentially difficult to overcomethe problem of environmental contamination associated with theemployment of the organic solvent.

Further, in the case of using a solvent type ink, the quality ofprinting is greatly influenced by the features of surface of printingmatter. For example, if the surface of printing mater is permeable,bleeding is more likely to occur, and if the surface of printing materis impermeable, it will become difficult to fix printed images.Moreover, since it will take a certain period of time in order to drycompletely an ink layer that has been formed on the surface of printingmatter, when an image is formed relatively thick on a large surface ofprinting matter, the image is more likely to collapse due to thefluidity of ink. Moreover, when the solvent type ink is employed, thereis high possibility that the printed surface is deteriorated in thedrying process of the ink layer. Thus, according to the aforementionedconventional techniques, it is not necessarily easy to obtain a printermatter with high qualities.

Incidentally, a photosensitive ink and a printer system employing thephotosensitive ink are now taken notice of as a technique for copingwith the aforementioned problem of solvent. The photosensitive ink to beemployed this case typically comprises a radical polymeric monomer, aphotopolymerization initiator and a pigment. Once this photosensitiveink is delivered onto a printing surface, it can be quickly photo-cured.

According to this technique, since the ink layer that has been formed ona printing surface can be non-fluidized by the irradiation of light, itis possible to obtain a printed matter which is relatively excellent inquality. However, this photosensitive ink includes a large amount ofcarcinogenic radical-generating agents such as a radical generatingagent and still more, a volatile acrylic acid derivative to be employedas a radical polymeric monomer is highly stimulating to skin and badlysmelling. Namely, it is necessary to take care in handling aphotosensitive ink of this kind. Further, the radical polymerization isconsiderably obstructed by the presence of oxygen in air atmosphere andadditionally, the light for exposure will be lost due to the absorptionof light by the pigment included in the ink. As a result, underexposuremay occur at a deep region of ink layer, so that the conventionalradical polymeric ink is poor in sensitivity to light. Accordingly, inorder to obtain a printed matter of high quality by using thisconventional technique, it would become necessary to construct a largescale exposure system.

There has been proposed, as an ink which is relatively free from theinfluence of oxygen, a photosensitive ink which is cationphoto-polymerizable. However, since the conventional ink of this typecontains a solvent, the problem of the release of the solvent intoenvironment cannot be overcome. Moreover, a cured layer is more likelyto be further hardened, thus giving rise to the problem of clogging ofthe delivery head since this hardened layer is insoluble. Furthermore,there has been proposed a cation-cure type photosensitive compositionwhich is adapted for inkjet delivery and can be employed for the coatingof CD-ROM. The composition which has been found capable of performingthe inkjet delivery mainly comprises vinyl ether and bisphenol A typeepoxy, and hence there is still a serious problem associated with therelease of these compounds to the environment.

There has been also proposed an inkjet ink having a specific compositionincluding a cation-curable monomer. Since this ink also contains, as anessential component, a specific kind of vinyl ether compound which ishigh in volatility, there is the same problem as described above.Further, ordinary vinyl ether compounds are accompanied with the problemthat when it is employed in combination with pigment, they become poorin polymerizability.

Additionally, there has been disclosed a photosensitive inkjet inkcomprising a polyvalent vinyl ether compound and an alicyclic epoxycompound. The vinyl ethers disclosed are accompanied with variousproblems that they are poor in polymerizability, that the skeleton ofbisphenol A exhibits its specific carcinogenic property, and that sincethe vinyl ethers are employed in combination with an alicyclic epoxywhich is high in solubility and dark-reactivity, they are poor in shelflife and a printed matter to be obtained is not sufficiently resistiveto a solvent.

In particular, when the printing surface is constituted by an absorptivemedium, there will be raised the problem that the conventional acrylicphoto-curable inkjet ink cannot be easily cured in the interior of theabsorptive printing surface.

Incidentally, the conventional photo-cation-cure type inkjet ink isaccompanied with the problem that the viscosity thereof can be greatlyfluctuated. The reason for this can be attributed to the fact that oncean acid generates due to changes with time of ink, the acid cannot beeasily deactivated, thus giving rise to a great magnitude of darkreaction of ink. In the case of inkjet ink, when the viscosity thereofis caused to change, it will lead to the disturbance of the ejectingshape of ink, to poor reproducibility of printing, and in the worstcase, to a critical condition such as defective delivery of ink andclogging of ink. Therefore, this problem is very serious.

BRIEF SUMMARY OF THE INVENTION

Therefore, one of the objects of the present invention is to provide aninkjet ink wherein the employment of an organic solvent is no longerrequired, and the provision of an exposure system of large scale forobtaining printed matters of high quality is no longer required. Anotherobject of the present invention is to provide an inkjet ink which iscapable of overcoming the problems such as poor cation polymerizingreaction and solvent resistance which are particularly accompanied withthe conventional photosensitive inkjet ink containing, as a maincomponent, a vinyl ether compound.

According to one aspect of the present invention, there is provided aninkjet ink comprising:

-   -   a photo-acid generating agent which is capable of generating an        acid as it is irradiated with light;    -   a color component; and    -   an acid-polymerizable compound which can be polymerized in the        presence of an acid;    -   wherein the acid-polymerizable solvent comprises a compound        represented by the following general formula (1):        R¹³—R¹⁴—(R¹³)_(p)   (1)

(Wherein R¹³(s) is a group selected from the group consisting of a vinylether group, a group having a vinyl ether skeleton, an alkoxy group,substituted hydroxyl group and hydroxyl group wherein at least one ofR¹³(s) is vinyl ether group or a group having a vinyl ether skeleton;R¹⁴ is a group comprising a substituted or unsubstituted cyclic skeletonand having a valence of (p+1); and p is a positive integer includingzero).

According to another aspect of the present invention, there is providedan inkjet ink comprising:

-   -   a photo-acid generating agent which is capable of generating an        acid as it is irradiated with light;    -   a color component; and    -   an acid-polymerizable compound which can be polymerized in the        presence of an acid;    -   wherein at least 40% of the acid-polymerizable solvent is a        compound represented by the following general formula (1), and        the viscosity of the ink is not higher than 12 mPa.sec at the        ordinary temperature:        R¹³—R¹⁴—(R¹³)_(p)   (1)

(Wherein R¹³(s) is a group selected from the group consisting of a vinylether group, a group having a vinyl ether skeleton, an alkoxy group,substituted hydroxyl group and hydroxyl group wherein at least one ofR¹³(s) is vinyl ether group or a group having a vinyl ether skeleton;R¹⁴ is a group comprising a substituted or unsubstituted cyclic skeletonand having a valence of (p+1); and p is a positive integer includingzero).

According to further aspect of the present invention, there is providedan inkjet ink comprising:

-   -   a photo-acid generating agent which is capable of generating an        acid as it is irradiated with light;    -   a color component; and    -   an acid-polymerizable compound which can be polymerized in the        presence of an acid;    -   wherein at least 40% of the acid-polymerizable solvent is a        compound represented by the following general formula (1), and        the ink further comprising an epoxy compound and/or oxetane        compound both having a solubility parameter of MPa^(1/2) or        more:        R¹³—R¹⁴—(R¹³)_(p)   (1)

(Wherein R¹³(s) is a group selected from the group consisting of a vinylether group, a group having a vinyl ether skeleton, an alkoxy group,substituted hydroxyl group and hydroxyl group wherein at least one ofR¹³(s) is vinyl ether group or a group having a vinyl ether skeleton;R¹⁴ is a group comprising a substituted or unsubstituted cyclic skeletonand having a valence of (p+1); and p is a positive integer includingzero).

According to further aspect of the present invention, there is providedan inkjet ink comprising:

-   -   a photo-acid generating agent which is capable of generating an        acid as it is irradiated with light;    -   a color component; and    -   an acid-polymerizable compound which can be polymerized in the        presence of an acid;    -   wherein at least 40% of the acid-polymerizable solvent is a        compound represented by the following general formula (1), and        the ink further comprising an epoxy compound and/or oxetane        compound both having a viscosity exceeding 500 mPa.sec at the        ordinary temperature:        R¹³—R¹⁴—(R¹³)_(p)   (1)

(Wherein R¹³(s) is a group selected from the group consisting of a vinylether group, a group having a vinyl ether skeleton, an alkoxy group,substituted hydroxyl group and hydroxyl group wherein at least one ofR¹³(s) is vinyl ether group or a group having a vinyl ether skeleton;R¹⁴ is a group comprising a substituted or unsubstituted cyclic skeletonand having a valence of (p+1); and p is a positive integer includingzero).

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by theinstrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the embodiments given below, serve toexplain the principles of the invention.

FIG. 1 is a schematic view of a recording apparatus making it possibleto employ an inkjet ink according to one embodiment of the presentinvention; and

FIG. 2 is a schematic view of a recording apparatus making it possibleto employ an inkjet ink according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Next, various embodiments of the present invention will be explained asfollows.

The inkjet ink according to one embodiment of the present inventioncomprises a photo-acid generating agent which is capable of generatingan acid as it is irradiated with light, a color component, and anacid-polymerizable compound which can be polymerized in the presence ofan acid. The inkjet ink according to this embodiment is formed of achemical amplification type photosensitive composition. The term “inkjetink” herein means an ink which is fluid at the ordinary temperature,more specifically an ink having a viscosity of 50 mPa.sec or less, morepreferably 30 mPa.sec or less at a temperature of 25° C. Alternatively,the inkjet ink may be an ink having a viscosity of 12 mPa.sec or less ata temperature ranging from the ordinary temperature to about 50° C.,i.e. within the temperature controlling range of an inkjet head.

When this inkjet ink is irradiated with light, an acid generates fromthe photo-acid generating agent, and then, this acid acts as a catalystfor the crosslinking reaction of the polymeric compound and at the sametime, diffuses inside the ink layer. Therefore, in this ink layer afterthe exposure thereof, the crosslinking reaction using this acid as acatalyst proceeds. This crosslinking reaction can be accelerated byapplying heat to the ink layer. In different from the radicalpolymerization, this crosslinking reaction would not be obstructed bythe presence of oxygen. As a result, a plurality of crosslinkingreactions occur using only one photon, thereby making it possible torealize a high sensitivity. Moreover, the crosslinking reaction can takeplace quickly even in a deep portion of the ink layer and in theinterior of the medium. Therefore, as compared with the case of radicalpolymerization system, the ink layer to be obtained in this manner wouldbecome far more excellent in adhesion.

Next, the components to be included in the inkjet ink according to theembodiments of the present invention will be explained in detail,respectively.

(Photo-Acid Generating Agent)

As for the examples of photo-acid generating agent which are capable ofgenerating acid as they are irradiated with light, it is possible toemploy onium salt, diazonium salt, quinone diazide compounds, organichalide compounds, aromatic sulfonate compounds, bisulfone compounds,sulfonyl compounds, sulfonate compounds, sulfonium compounds, sulfamidecompounds, iodonium compounds, sulfonyl diazomethane compounds andmixtures of these compounds.

Specific examples of the aforementioned compounds includetriphenylsulfonium triflate, diphenyliodonium triflate,2,3,4,4-tetrahydroxybenzophenone-4-naphthoquinone diazide sulfonate,4-N-phenylamino-2-methoxyphenyl diazonium sulfate,4-N-phenylamino-2-methoxyphenyldiazonium-p-ethylphenyl sulfate,4-N-phenylamino-2-methoxyphenyldiazonium-2-naphthyl sulfate,4-N-phenylamino-2-methoxyphenyldiazonium-phenyl sulfate,2,5-diethoxy-4-N-4′-methoxyphenylcarbonylphenyldiazonium-3-carboxy-4-hydroxyphenyl sulfate,2-methoxy-4-N-phenylphenyldiazonium-3-carboxy-4-hydroxyphenyl sulfate,diphenylsulfonyl methane, diphenylsulfonyl diazomethane, diphenyldisulfone, α-methylbenzoin tosylate, pyrogallo trimesylate, benzointosylate, MPI-103 (CAS.NO.[87709-41-9]; Midori Kagaku Co., Ltd.),BDS-105 (CAS.NO.[145612-66-4]; Midori Kagaku Co., Ltd.), NDS-103(CAS.NO.[110098-97-0]; Midori Kagaku Co., Ltd.), MDS-203(CAS.NO.[127855-15-5]; Midori Kagaku Co., Ltd.), Pyrogallo tritosylate(CAS.NO.[20032-64-8]; Midori Kagaku Co., Ltd.), DTS-102(CAS.NO.[75482-18-7]; Midori Kagaku Co., Ltd.), DTS-103(CAS.NO.[71449-78-0]; Midori Kagaku Co., Ltd.), MDS-103(CAS.NO.[127279-74-7]; Midori Kagaku Co., Ltd.), MDS-105(CAS.NO.[116808-67-4]; Midori Kagaku Co., Ltd.), MDS-205(CAS.NO.[81416-37-7]; Midori Kagaku Co., Ltd.), BMS-105(CAS.NO.[149934-68-9]; Midori Kagaku Co., Ltd.), TMS-105(CAS.NO.[127820-38-6]; Midori Kagaku Co., Ltd.), NB-101(CAS.NO.[20444-09-1]; Midori Kagaku Co., Ltd.), NB-201(CAS.NO.[4450-68-4]; Midori Kagaku Co., Ltd.), DNB-101(CAS.NO.[114719-51-6]; Midori Kagaku Co., Ltd.), DNB-102(CAS.NO.[131509-55-2]; Midori Kagaku Co., Ltd.), DNB-103(CAS.NO.[132898-35-2]; Midori Kagaku Co., Ltd.), DNB-104(CAS.NO.[132898-36-3]; Midori Kagaku Co., Ltd.), DNB-105(CAS.NO.[132898-37-4]; Midori Kagaku Co., Ltd.), DAM-101(CAS.NO.[1886-74-4]; Midori Kagaku Co., Ltd.), DAM-102(CAS.NO.[28343-24-0]; Midori Kagaku Co., Ltd.), DAM-103(CAS.NO.[14159-45-6]; Midori Kagaku Co., Ltd.), DAM-104(CAS.NO.[130290-80-1] and CAS.NO.[130290-82-3]; Midori Kagaku Co.,Ltd.), DAM-201 (CAS.NO.[28322-50-1]; Midori Kagaku Co., Ltd.), CMS-105(Midori Kagaku Co., Ltd.), DAM-301 (CAS.NO.[138529-81-4]; Midori KagakuCo., Ltd.), SI-105 (CAS.NO.[34694-40-7]; Midori Kagaku Co., Ltd.),NDI-105 (CAS.NO.[133710-62-0]; Midori Kagaku Co., Ltd.); EPI-105(CAS.NO.[135133-12-9]; Midori Kagaku Co., Ltd.); and UVACURE1591(DAICELUCB Co., Ltd.).

Further, as the photo-acid generating agent, the following compounds canbe also employed.

In the above general formulas, C₁ and C₂ represent respectively carbonatom forming a single bond or a double bond; R₁₀ is hydrogen atom,fluorine atom, alkyl group or aryl group; and R₁₁ and R₁₂ representrespectively a monovalent organic group and may be linked to each otherto form a cyclic structure.

Further, as the photo-acid generating agent, the following compounds canbe also employed.

(In the above general formulas, Z is alkyl group.)

Among these compounds, preferable examples of the photo-acid generatingagents are onium salts. Examples of onium salts useful in this case arediazonium salts, phosphonium salts and sulfonium salts having, as acounter ion, fluoroboric acid anion, hexafluoroantimonic acid anion,hexafluoroarsenic acid anion, trifluoromethane sulfonate anion,paratoluene sulfonate anion or paranitrotoluene sulfonate anion. Inparticular, it is especially preferable that the photo-acid generatingagent comprises onium salts or triazine halide compounds represented bythe following general formulas (4) and (5). These photo-acid generatingagents are advantageous in terms of both sensitivity and stability.

In this general formula, R6, R7, R8, R9 and R10 individually representseither aromatic group or functional group having a chalcogenide atom andan aromatic group; C1 and C2 individually represents a chalcogenideatom; A4 and A5 individually represents anion species selected from thegroup consisting of PF₆ ⁻, SbF₆ ⁻, BF₄ ⁻, AsF₆ ⁻, CF₃SO₃ ⁻, C₄F₉SO₃ ⁻and CH₃SO₃ ⁻; and m and n individually represents an integer.Incidentally, the term “chalcogenide atom” means a chalcogen atom andother atoms which are more positive than the chalcogen atoms. Further,“chalcogen atom” means to include sulfur, selenium, tellurium, poloniumand iodine atoms.

The onium salts represented by the aforementioned general formulas (4)and (5) are high in curing reactivity and excellent in stability at theordinary temperature. Therefore, the onium salts are capable ofsuppressing the curing of the aforementioned inkjet ink under theconditions where light is not irradiated thereto.

If the compounds represented by the aforementioned general formulas (4)and (5) are to be employed as a photo-acid generating agent, theaforementioned chalcogenide atom should preferably be selected fromsulfur atom and iodine atom in view of securing the thermal stability ofthe photo-acid generating agent and the stability thereof to water. Inthis case, the anion species should preferably be formed of anon-organic acid, in particular, formed of PF₆ ⁻ in view of securingsuitable acidity and thermal stability. Further, it is especiallypreferable to employ hexafluorophosphate compounds having aphenylsulfonium skeleton in view of concurrently improving thephotosensitivity of the photo-acid generating agent.

The photo-acid generating agent may further contain, if required, asensitizing dye. Examples of such a sensitizing dye include acridinecompounds, benzofuravins, perylene, anthracene and laser dyes.

When quinone diazide compounds are to be employed as a photo-acidgenerating agent, salts thereof such as naphthoquinone diazide sulfonylchloride or naphthoquinone diazide sulfonic acid may be employed.

Organic halide means a compound which is capable of forming hydroacidhalide, specific examples thereof being set forth for example in U.S.Pat. No. 3,515,552; U.S. Pat. No. 3,536,489; and U.S. Pat. No.3,779,778; and in German Patent Laid-Open Publication No. 2243621.Specifically, U.S. Pat. No. 3,515,552 describes carbon tetrabromide,tetra(bromomethyl) methane, tetrabromoethylene,1,2,3,4-tetrabromobutane, trichloroethoxy ethanol, p-iodophenol,p-bromophenol, p-iodobiphenyl, 2,6-dibromophenol, 1-bromo-2-naphthol,p-bromoaniline, hexachloro-p-xylene, trichloroacetoanilide,p-bromodimethyl aniline, tetrachlorotetrahydronaphthalene,α,α′-dibromoxylene, α,α,α′,α′-tetrabromoxylene, hexabromoethane,1-chloroanthraquinone, ω,ω,ω-tribromoquinalizine, hexabromocyclohexane,9-bromofluorene, bis(pentachloro) cyclopentadiphenyl, polyvinylidenechloride and 2,4,6-trichlorophenoxyethyl vinylether. U.S. Pat. No.3,779,778 describes hexabromoethane, α,α,α-trichloroacetophenone,tribromotrichloroethane and halomethyl-S-triazine. Among them,halomethyl-S-triazine such as2,4-bis(trichloromethyl)-6-methyl-S-triazine and2,4,6-tris(trichloromethyl)-S-triazine are preferable for use. Morepreferable examples of the organic halide compound include those whichare substituted by vinylhalomethyl-S-triazine, which is disclosed inU.S. Pat. No. 3,987,037. This vinylhalomethyl-S-triazine compound is aphoto-decomposable S-triazine having at least one trihalomethyl groupand a group which is conjugated through at least one ethylenicallyunsaturated bond with triazine ring. This vinylhalomethyl-S-triazinecompound can be represented by the following formula (A).

Wherein Q represents bromine atom or chlorine atom; P is —CQ₃, —NH₂,—NHR, —NR₂ or —OR; R is phenyl or lower alkyl having not more than 6carbon atoms; n is an integer ranging from 1 to 3; and W is an aromaticring, a heterocycle or a group represented by the following generalformula (B):

Wherein Z is oxygen atom or sulfur atom; and R1 is lower alkyl group orphenyl group.

At least one hydrogen atom in the aromatic ring or heterocyclerepresented by W in the above general formula (A) may be furthersubstituted. In this case, examples of the substituent group include,for example, chlorine atom, bromine atom, phenyl group, lower alkylgroup having not more than 6 carbon atoms, nitro group, phenoxy group,alkoxy group, acetoxy group, acetyl group, amino group and alkylaminogroup. As for specific examples of vinylhalomethyl-S-triazine compound,they include the following groups.

In addition to the aforementioned compounds, it is also possible tosuitably employ, as a photo-acid generating agent, a compound havingtriazine ring having trihalomethane introduced into the skeletonthereof. If this triazine ring is provided with four or more conjugateddouble bonds, the photosensitive wavelength of the photo-acid generatingagent would be shifted to the longer wavelength side. Therefore, if anordinary high-pressure mercury lamp is to be employed as a light source,the employment of such a compound would be preferable. Specific examplesof such a compound include triazine having naphthalene substituentgroup, and condensed triazine compounds.

Further, it is also possible to suitably employ acid esters havingphotodissociating property as a photo-acid generating agent. Specificexamples of such esters include orthonitrobenzyl ester of alumi silanol.

The mixing ratio of the photo-acid generating agent in the inkjet inkmay be suitably selected depending on the acid generating efficiency ofthe photo-acid generating agent to be employed as well as on thequantity of the color component. For example, when the concentration ofthe pigment in the inkjet ink is about 5% by weight, the mixing ratio ofthe photo-acid generating agent may be confined generally within therange of 1 to 10 parts by weight, more preferably 2 to 8 parts byweight, most preferably 4 to 8 parts by weight per 100 parts by weightof a compound which is polymerizable in the presence of an acid andincluded in the inkjet ink. Especially when the concentration of thepigment in the inkjet ink is confined within this more preferable range,the storage stability of the ink can be enhanced and at the same time,the corrosion of the piping or head member of recording apparatus can beminimized. If the mixing ratio of the photo-acid generating agent isless than 1 part by weight per 100 parts by weight of theacid-polymerizable compound, the sensitivity of the inkjet ink would bedeteriorated. On the other hand, if the mixing ratio of the photo-acidgenerating agent exceeds 10 parts by weight, increase in viscosity withtime of the ink would be intensified thereby deteriorating the coatingproperties of the ink and lowering the hardness of the ink film that hasbeen photo-cured. Additionally, the corrosion of the piping or headmember of recording apparatus may occur.

(Color Component)

As for the color component to be employed in the present invention, itis possible to employ pigment and/or dye. However, since an acid isemployed in the mechanism in the case of the inkjet ink according to theembodiments of the present invention, it is more preferable to employpigment rather than dye which can be more easily faded away by the acid.

As for the pigments useful as a color component in this case, there isnot any particular limitation as long as they have optical coloring andtinting properties demanded of pigments. The pigments to be employed inthis case may be further provided with other properties such asmagnetism, fluorescence, conductivity, dielectric property, etc. inaddition to the coloring and tinting properties. If the pigments areprovided with these various properties, it may become possible to obtainan image having various functions. Further, the pigments may containparticles which are effective in providing an ink layer with increasedheat resistance or physical strength.

As for the examples of pigments useful in this case, they includephotoabsorptive pigments for example. Specific examples of suchphotoabsorptive pigments include carbonaceous pigment such as carbonblack, carbon refined and carbon nanotube; metal oxide pigments such asiron black, cobalt blue, zinc oxide, titanium oxide, chromium oxide andiron oxide; sulfide pigments such as zinc sulfide; phthalocyaninepigments; pigments formed of salts such as metal sulfate, metalcarbonate, metal silicate and metal phosphate; and pigments formed ofmetal powder such as aluminum powder, bronze powder and zinc powder.

Further, it is also possible to employ organic pigments including, forexample, dye chelate; nitro pigments; nitroso pigments such as anilineblack and naphthol green B; azo pigments such as Bordeaux 10B, Lake red4R and chromophthal red (including azo lake, insoluble azo pigment,condensed azo pigment, chelate azo pigment); Lake pigments such asPeacock blue lake and Rhodamine lake; phthalocyanine pigments such asphthalocyanine blue; polycyclic pigments (such as perylene pigment,perinone pigment, anthraquinone pigment, quinacridone pigment, dioxanepigment, thioindigo pigment, isoindolinone pigment, quinophthalenepigment, etc.); threne pigments such as thioindigo red and indanthroneblue; quinacridone pigment; quinacridine pigment; and isoindolinonepigment.

As for the pigments that can be employed in the manufacture of a blackink, it is possible to employ carbon black such as Raven 5750, Raven5250, Raven 5000, Raven 3500, Raven 1255 and Raven 700 (all availablefrom Colombia Co., Ltd.); Regal 400R, Regal 330R, Regal 660R, Mogul L,Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000,Monarch 1100, Monarch 1300 and Monarch 1400 (all available from CabotCo., Ltd.); No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52,MA7, MA8, MA100 and No. 2200B (all available from Mitsubishi ChemicalCo., Ltd.); Color Black FW1, Color Black FW2, Color Black FW2V, ColorBlack FW18, Color Black FW200, Color Black S150, Color Black S160, ColorBlack S170, Printex 35, Printex U, Printex V, Printex 140U, SpecialBlack 6, Special Black 5, Special Black 4A and Special Black 4 (allavailable from Dexa Co., Ltd.).

As for the yellow pigments that can be employed in a yellow ink,examples thereof include Yellow 128, C.I. Pigment Yellow 129, C.I.Pigment Yellow 139, C.I. Pigment Yellow 151, C.I. Pigment Yellow 150,C.I. Pigment Yellow 154, C.I. Pigment Yellow 1, C.I. Pigment Yellow 2,C.I. Pigment Yellow 3, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13,C.I. Pigment Yellow 14C, C.I. Pigment Yellow 16, C.I. Pigment Yellow 17,C.I. Pigment Yellow 73, C.I. Pigment Yellow 74, C.I. Pigment Yellow 75,C.I. Pigment Yellow 83, C.I. Pigment Yellow 93, C.I. Pigment Yellow 95,C.I. Pigment Yellow 97, C.I. Pigment Yellow 98, C.I. Pigment Yellow 114,and Pigment Yellow 180. In particular, it is more preferable, amongthese yellow pigments, to employ Pigment Yellow 180, since it can behardly discolored by the effect of oxygen.

As for the pigments that can be employed in a magenta ink, examplesthereof include C.I. Pigment Red 123, C.I. Pigment Red 168, C.I. PigmentRed 184, C.I. Pigment Red 202, C.I. Pigment Red 5, C.I. Pigment Red 7,C.I. Pigment Red 12, C.I. Pigment Red 48(Ca), C.I. Pigment Red 48(Mn),C.I. Pigment Red 57(Ca), C.I. Pigment Red 57:1, C.I. Pigment Violet 19and C.I. Pigment Red 112.

Further, as for the pigments that can be employed in a cyanine ink,examples thereof include C.I. Pigment Blue 15:3, C.I. Pigment Blue15:34, C.I. Pigment Blue 16, C.I. Pigment Blue 22, C.I. Pigment Blue 60,C.I. Pigment Blue 1, C.I. Pigment Blue 2, C.I. Pigment Blue 3, C.I. VatBlue 4, and C.I. Vat Blue 60.

Further, it is also useful, as a color component, to employ whitepigments such as natural clay, white lead, zinc white and metalcarbonates such as magnesium carbonate, metal oxides such as barium andtitanium. The inkjet ink containing white pigments can be employed notonly in white printing but also in the amendments of printing orunderlying images through overwriting.

As for fluorescent pigments, it is possible to employ not only inorganicfluorescence materials but also organic fluorescence materials. As forthe inorganic fluorescence materials, specific examples of which includeMgWO₄, CaWO₄, (Ca, Zn) (PO₄)₂:Ti⁺, Ba₂P₂O₇:Ti, BaSi₂O₅:Pb²⁺,Sr₂P₂O₇:Sn²⁺, SrFB₂O_(3.5):Eu²⁺, MgAl₁₆O₂₇:Eu²⁺, and inorganic acidsalts such as tungstenate and sulfate. As for the organic fluorescencematerials, specific examples of which include acridine orange, aminoacridine, quinacrine, anilinonaphthalene sulfonate derivatives, anthroyloxystearic acid, auramine O, chlorotetracycline, cyanine dye such asmerocyaninen and 1,1′-dihexyl-2,2′-oxacarboxycyanine, dansylsulfonamide, dansyl choline, dansyl galactoside, dansyl tolidine, dansylchloride derivatives such as dansyl chloride, diphenyl hexatriene,eosin, ε-adenosine, ethidium bromide, fluorescein, foamycine,4-benzoylamide-4′-aminostilbene-2,2′-sulfonic acid, β-naphthyltriphosphic acid, oxonol dye, parinaric acid derivatives, perylene,N-phenylnaphthyl amine, pyrene, safranine O, fluorescamine, fluoresceinisocyanate, 7-chloronitrobenzo-2-oxa-1,3-diazole, dansylaziridine,5-(iodoacetamide ethyl) aminonaphthalene-1-sulfonic acid,5-iodofluorescein, N-(1-anilinonaphthyl 4) maleimide,N-(7-dimethyl-4-methylcumanyl) maleimide, N-(3-pyrene) maleimide,eosin-5-iodoacetamide, fluorescein mercury acetate,2-[4′-(2″-iodoacetamide)]aminonaphthalene-6-sulfonic acid, eosin,Rhodamine derivatives, organic EL dye, organic EL polymer, organic ELcrystal and dendrimer.

As for the powder to be employed for enhancing the heat resistance andphysical strength of ink layer, examples of which include oxides ornitrides of aluminum and silicon, filler and silicon carbide. For thepurpose of providing the ink layer with electric conductivity, the inkmay further contain conductive carbon pigment, carbon fiber, or powderof copper, silver, antimony and other noble metals. Iron oxide powderand ferromagnetic powder are suited for use in providing the ink layerwith magnetic property. It is also possible to incorporate metal oxidepowder such as tantalum oxide or titanium oxide exhibiting highdielectricity the recording ink.

It is also possible to incorporate dyes as an auxiliary component ofpigment into the inkjet ink according to this embodiment. For example,dyes which are low in acidity and basicity and excellent in solubilityto a solvent, such as azoic dye, sulfur (building materials) dye,disperse dye, fluorescent brightening agent and oil soluble dye can beemployed. Among them, it is more preferable to employ oil soluble dyesuch as azo dye, triaryl methane dye, anthraquinone dye and azine dye.Specific examples of such oil soluble dye include C.I. Slovent Yellow-2,6, 14, 15, 16, 19, 21, 33, 56, 61 and 80; Diaresin Yellow-A, F, GRN andGG; C.I. Slovent Violet-8, 13, 14, 21 and 27; C.I. Disperse Violet-1;Sumiplast Violet RR; C.I. Slovent Blue-2, 11, 12, 25 and 35; DiresinBlue-J, A, K and N; Orient Oil Blue-IIN, #603; and Sumiplast Blue BG.

These pigments and dyes described above may be employed singly or incombination of two or more. Further, for the purpose of enhancing thephotoabsorbance, saturation and color vision, pigments and dyes can beemployed in combination. Furthermore, in order to enhance thedispersibility of the pigments, these pigments may be subjected totreatments for enabling the pigments to bond with a high molecularbinder or for micro-capsulizing the pigments.

The content of the color component should be confined within the rangeof 1 to 25% by weight based on the ink. If the content of the pigmentsis less than 1% by weight, it would become difficult to secure asufficient color density. On the other hand, if the content of the colorcomponent is increased over 25% by weight, the delivery of the ink wouldbe deteriorated. Therefore, the content of the color component shouldmore preferably be confined within the range of 2 to 8% by weight basedon the ink.

Further, the content of the powder components in the ink shouldpreferably be confined within the range of 1 to 50% by weight. If thecontent of the powder components is less than 1% by weight, it wouldbecome difficult to expect sufficient effects of enhancing thesensitivity of the ink. On the other hand, if the content of the powdercomponents is higher than 50% by weight, the resolution and sensitivityof the ink would be deteriorated.

The average particle diameter of the color component and of the powdercomponents in the inkjet ink should be as small as possible. Generally,the average particle diameter of the color component and of the powdercomponents is confined to not larger than ⅓, more preferably not largerthan about {fraction (1/10)} of the diameter of the opening of nozzlethrough which the inkjet ink is discharged. Incidentally, the diameterof the opening of nozzle is typically 10 μm or less, more preferably 1μm or less. Therefore, when the settling of the color component andpowder components is taken into consideration, a preferable particlediameter of the color component and of the powder components should be0.35 μm or less as these components are to be employed as an ink forinkjet printing. Therefore, the average particle diameter of thesecomponents should be usually confined within the range of 0.1 to 0.3 μm.

(Compounds Polymerizable in the Presence of an Acid)

The compounds which are polymerizable in the presence of an acid can beemployed as a solvent in the inkjet ink according to the embodiments ofthe present invention. This solvent is substantially formed of apolymerizable compound having such a feature. This expression of“solvent is substantially formed of a polymerizable compound” isintended to include not only a situation wherein “solvent is formed ofonly a polymerizable compound” but also a situation wherein “solvent isformed of a polymerizable compound and a minute amount of impuritieswhich are unavoidably intermingled in the solvent”. This “a minuteamount of impurities which are unavoidably intermingled in the solvent”may be existed in the solvent at a concentration of, at most, not morethan 10% by weight based a total weight of the solvent. More preferably,the content of “impurities which are unavoidably intermingled in thesolvent” should be confined to not more than 5% by weight in general. Ifthe content of impurities exceeds this upper limit, when residualimpurities in the solvent may fly into air atmosphere, raising theproblem of environmental safety or residual impurities may remain insidea cured material, thus deteriorating the curing property of the ink.Next, the components of the ink will be classified and explained indetail.

Vinylether Compounds (Essential Component)

The inkjet ink according to the embodiments of the present inventionessentially comprises a vinyl ether compound. Further, at least 40% byweight of the acid-polymerizable compound should be occupied by a vinylether compound. If the aforementioned solvent is solely constituted by avinyl ether compound, the content of the vinyl ether compound shouldpreferably be confined to 30% by weight or more, more preferably 40% byweight or more based on a total weight of the inkjet ink. When thecontent of the vinyl ether compound is less than 30% by weight, theclogging of nozzle may generate or the photosensitivity of the ink maybe deteriorated.

The vinyl ether compound should preferably be provided with a cyclicskeleton in viewpoint of polymerizability and curing hardness. Morespecifically, preferable examples of the vinyl ether compound are thoserepresented by the following general formula (1).R¹³—R¹⁴—(R¹³)_(p)   (1)

Wherein R¹³(s) is a group selected from vinyl ether group, a grouphaving a vinyl ether skeleton, alkoxy group, hydroxyl-substituted groupand hydroxyl group wherein at least one of R¹³(s) is vinyl ether groupor a group having a vinyl ether skeleton; R¹⁴ is a group having asubstituted or unsubstituted cyclic skeleton and a valence of (p+1); andp is a positive integer including zero. If R¹⁴ is a cyclohexane cyclicskeleton and p is zero, it is preferable, in viewpoint of volatility, toemploy the cyclic skeleton which is constituted by an oxygen-containingstructure. More specifically, at least one of carbon atoms located onthe ring should be constructed into a structure having a ketonestructure, a structure where the carbon atom is substituted by oxygenatom, or a structure having an oxygen-containing substituent group.

The number of vinyl ether group to be introduced into the aforementionedmolecular skeleton should preferably be as many as possible in view ofimproving curability. More specifically, although there is not anyparticular limitation, if it is desired to provide a cured ink layerwith resolubility, the number of vinyl ether group should preferably beconfined at most to about 2 or 3.

As for the specific examples of the cyclic group R¹⁴ having a valence of(p+1), it is possible to employ substituted or unsubstituted aromaticrings such as (p+1)-valent groups including benzene ring, naphthalenering and biphenyl ring. Alternatively, it is also possible to employ(p+1)-valent groups that can be derived from an alicyclic skeleton suchas cycloalkane skeleton, norbornane akeleton, adamantane skeleton,tricyclodecane skeleton, tetracyclododecane skeleton, terpenoid skeletonor cholesterol skeleton. When these alicyclic skeletons having acrosslinkage structure are employed, the hardness of the cured materialcan be enhanced and hence the employment of such alicyclic skeletons ispreferable. In viewpoint of volatility, it is preferable to employ thealicyclic skeleton which is constituted by an oxygen-containingstructure. More specifically, at least one of carbon atoms located onthe ring should be constructed into a structure having a ketonestructure, a structure where the carbon atom is substituted by oxygenatom, or a structure having an oxygen-containing substituent group.

The compounds represented by the aforementioned general formula (1)generally have a viscosity ranging from 1 mPa.s to about 30 mPa.s.Accordingly, the employment of these compounds are effective insufficiently reducing the viscosity of inkjet ink. For the purpose offulfilling this effect, the mixing ratio of low viscosity vinyl ethercompounds should be 40% by weight or more.

Among the compounds represented by the aforementioned general formula(1), those where at least one vinyl ether group is directly bonded tothe ring are preferable since they are excellent in cation curability,which excellent property can be retained even if pigment is concurrentlyexisted.

The employment of cyclic compounds comprising an aromatic skeleton ispreferable since they are capable of enhancing the hardness of curedmaterial or enhancing the solubility of a photosensitive agent, etc.Examples of such vinyl ether compounds include those where the hydroxylgroup of the following alcohol compounds is substituted by vinyl etheror propenyl ether. Examples of such compounds include alcohols such ascumene alcohol represented by the following chemical formula Aro.1,vinyloxybenzene represented by the following chemical formula Aro.2,hydroquinone represented by the following chemical formula Aro.3,1-carbomethoxy-4-vinyloxybenzene represented by the following chemicalformula Aro.4, 2-hydroxynaphthalene represented by the followingchemical formula Aro.5, 1-tert-butyl-4-vinyloxybenzene represented bythe following chemical formula Aro.6, bisphenol A represented by thefollowing chemical formula Aro.7, 1-octyl-4-vinyloxybenzene representedby the following chemical formula Aro.8, 1-hydroxy-3,5-dimethylbenzenerepresented by the following chemical formula Aro.9, 4-hydroxycumylphenol represented by the following chemical formula Aro.10, and3-isopropyl phenol represented by the following chemical formula Aro.11.

Following chemical formulas illustrate specific examples of such vinylether compounds.

When the cyclic compounds are constituted by an alicyclic skeleton, theyare more preferable than aromatic vinyl ethers in terms of odor andenvironmental safety. As for the vinyl ether compounds having such analicyclic skeleton, it is more preferable to employ those having amonocyclic skeleton constituted by a 4-6 member ring, or those where the4-6 member rings are bonded to each other, forming a crosslinkingstructure. Specific examples of such vinyl ether compounds are thosewhere the hydroxyl group of the following alicyclic alcohol compounds issubstituted by vinyl ether or propenyl ether for instance. Examples ofthe alicyclic alcohol compounds are cyclopentane mono(di)ol,cyclopentane mono(di)methanol, cyclohexane mono(di)ol, cyclohexanemono(di)methanol, norbornane mono(di)ol, norbornane monoolmonomethanol,norbornane mono(di)methanol, tricyclodecane mono(di)ol, tricyclodecanemono(di)methanol, adamantane mono(di)ol, etc.

More specifically, aforementioned alicyclic skeletons have structuresrepresented by the following general formulas (VE1-a) and (VE1-b).

(Wherein X1 and Z1 are respectively alkylene group having 1 to 5 carbonatoms; Y1 is alkylene group having 1 to 2 carbon atoms; and k is 0 or1.)

Specific examples of the aforementioned compounds are those where thehydroxyl group of the following alcohol compounds is substituted byvinyl ether or propenyl ether. Examples of such alcohol compounds are,for instance, 4-cyclohexane diol represented by the following chemicalformula Ali.1, dicyclopentadiene methanol represented by the followingchemical formula Ali.2, isoborneol represented by the following chemicalformula Ali.3, 1-tert-butyl-4-vinyloxycyclohexanol represented by thefollowing chemical formula Ali.4, trimethylcyclohexanol represented bythe following chemical formula Ali.5, dihydroxyoctahydrophenylrepresented by the following chemical formula Ali.6,hydroxytricyclodecanemonoene represented by the following chemicalformula Ali.7, menthol represented by the following chemical formulaAli.8, 1,3-dihydroxycyclohexane represented by the following chemicalformula Ali.9, decahydro-2-naphthalenol represented by the followingchemical formula Ali.10, vinyloxycyclododecanol represented by thefollowing chemical formula Ali.11, norbornane diol represented by thefollowing chemical formula Ali.12, etc.

Following chemical formulas illustrate specific examples of such vinylether compounds.

Among the aforementioned compounds, those having an alicyclic skeletonof bridged structure is more preferable since they are capable ofenhancing the hardness of cured material. Although there are also knownRAPI-CURE CHVE: cyclohexane dimethanoldivinyl ether and RAPI-CURE CHMVE:cyclohexane dimethanolmonovinyl ether (all available from ISP Japan Co.,Ltd.), since these compounds are constructed such that vinyl ether isnot directly bonded to a cyclic skeleton, they are liable to become poorin terms of acid-polymerizability.

Among the compounds having the aforementioned alicyclic skeleton, thosehaving an oxygen-containing structure such as a structure where one orsome of carbon atoms of the ring are substituted by oxygen atom, or astructure having an oxygen-containing substituent group are preferablein terms of volatility and pigment dispersibility.

As for the vinyl ether compounds having a ring structure having theaforementioned acid-containing substituent group, it is possible toemploy those having a monocyclic skeleton constituted by a 4-6 memberring, or those where the 4-6 member rings are bonded to each other,forming a crosslinking structure. More specifically, it is possible toemploy vinyl ether compounds which are formulated such that, forexample, at least one of hydroxyl groups of alcohol compounds issubstituted by ether or ester such as methoxy group, methoxyethoxygroup, alkoxy group, acetoxy group or alkylester group, and the rest issubstituted by vinyl ether or propenyl ether. Examples of such alcoholcompounds include cyclopentane diol, cyclohexane di(tri)ol, cyclohexanedi(tri)methanol, norbornane di(tri)ol, norbornane mono(di)olmono(di)methanol, norbornane di(tri)methanol, tricyclodecane di(tri)ol,tricyclodecane di(tri)methanol, adamantane di(tri)ol, etc.

More specifically, vinyl ether compounds having oxygen-containingsubstituent group and represented by the following chemical formula aremost preferable.

On the other hand, vinyl ether compounds having oxygen atom includedinside the alicyclic skeleton is more preferable, since such compoundsare capable of further enhancing the stability of viscosity. Specificexamples such compounds are those represented by the following generalformula (VE2-a) or (VE2-b).

(Wherein at least one of X2, Y2 and Z1 contains at least one oxygenatom; X2 and Z2 are respectively alkylene group having 1 to 5 carbonatoms or bivalent organic group containing oxygen atom as ether linkage;Y2 is oxygen atom, alkylene group having 1 to 2 carbon atoms, orbivalent organic group containing oxygen atom as ether linkage; and k is0 or 1.)

These compounds described above are characterized by excellentenvironmental safety and curability which are peculiar to the alicyclicskeleton and by the inclusion of a cyclic hydrocarbon skeleton havingoxygen atom as a ring-constituting atom and exhibiting a high surfacetension, thus enabling them to have high solubility or dispersibility.Additionally, in order to minimize the phenomenon of run-away of ink bya relatively hydrophilic printing medium, the employment of the vinylether compounds provided with all of these features is most preferable.

As for the vinyl ether compounds having such an alicyclic skeleton asconstructed above, it is more preferable to employ those having a cyclicether skeleton constituted by a 4-6 member ring. Specific examples ofsuch vinyl ether compounds are those where the hydroxyl group of thefollowing alcohol compounds is substituted by vinyl ether or propenylether for instance. Examples of the alcohol compounds are substituted orunsubstituted oxetane monool, substituted or unsubstituted oxetanemonomethanol, oxapentane mono(di)ol, oxacyclohexane mono(di)ol,isosorbitol, mannitol, oxanorbornane mono(di)ol, oxanorbornanemonoolmonomethanol, oxanorbornane mono(di)methanol, oxatricyclodecanemono(di)ol, oxaadamantane mono(di)ol, and dioxorane methanol, etc.

Among these compounds, it is more preferable to employ those having astructure represented by the aforementioned general formulas (VE2-a) or(VE2-b) wherein a ratio between the number of oxygen atom and the numberof carbon atom (number of oxygen atom/number of carbon atom) is higherthan 0.08. It is possible, through the employment of these vinyl ethercompounds, to obtain an ink which is capable of exhibiting preferablephysical properties which are related to polarity such as solubility andwettability to a printing medium. The ratio of (number of oxygenatom/number of carbon atom) should preferably be not less than 0.15,more preferably not less than 0.25.

Specific examples of the vinyl ether compounds are CasNo. 22214-12-6 andCasNo. 20191-85-9. As exemplified by these compounds, compounds having adistorted cyclic ether structure such as oxetane ring or hydrofuran ringare preferable for use since such compounds are capable of enhancing thereactivity. Among them, compounds having hydrofuran ring is preferablein terms of volatility. Further, when these cyclic structures areconstructed to have a bridged structure, the curing hardness of ink canbe enhanced and hence compounds having such a structure are mostpreferable.

More specifically, the employment of the following vinyl ethers are mostpreferable.

A series of the aforementioned vinyl ether compounds can be suitablysynthesized by the methods described in J. Chem. Soc., 1965(2)1560-1561or in J. Am. Chem. Soc. vol. 124, No. 8, 1590-1591(2002). If thesemethods are to be employed, corresponding aromatic alcohols or alicyclicalcohols are employed as a raw material and reacted with acetic estersof vinyl ether or propenyl ether in the presence of a catalyst such asiridium halide. By using these methods, vinyl ether or propenyl etheraimed at can be easily manufactured. For example, menthol and vinylacetate are mixed together in a mixed solution comprising sodiumcarbonate and toluene with an iridium compound being employed as acatalyst under heating and in an argon atmosphere, thereby obtainingmenthol vinyl ether (MTVE).

These synthesizing methods can be suitably applicable to any kinds ofcompounds exemplified in this specification.

(Other Kinds of Acid-Polymerizable Compounds that Can be Added)

The employment singly of aforementioned vinyl ether compounds is mostpreferable in terms of curing property of ink. However, as long as thiscuring property is not prominently degraded, it is possible to employthem in combination with other kinds of compounds which arepolymerizable in the presence of an acid. Employing such compounds mayincrease storage stability or solubility of a photosensitizer, andprovid properties such as appropriate plasticity and glossiness.

As for the specific examples of the compounds which are polymerizable inthe presence of an acid and useful in this case, as long as they aregenerally known as an acid-polymerizable, it is possible to employwithout any particular limitation. For example, it is possible to employcompounds having a cyclic ether group such as epoxy group, oxetane groupand oxorane group; acrylic or vinyl compounds having the aforementionedsubstituent groups on their side chain; carbonate compounds; lowmolecular melamine compounds; vinyl carbazoles; styrene derivatives;alpha-methylstyrene derivatives; vinyl alcohol esters including estersof vinyl alcohol with acryl or methacryl; and monomers having a cationpolymerizable vinyl bond. These compounds can be employed singly or incombination of two or more. It is not necessarily possible to obtainexcellent properties in the employment of all of these compounds, but itis possible, only through a specific combination thereof for specificpurposes, to enhance other properties without deteriorating the curingproperty of ink. Details about this will be explained below.

(Epoxy Compound)

In addition to the vinyl ether compounds represented by theaforementioned general formula (1), it is possible to employ thefollowing compounds. Namely, it is possible to employ compoundsrepresented by the following general formula (11) or (12) andcharacterized by having a hydrocarbon group having a bivalent aliphaticor alicyclic skeleton each having 1 to 15 carbon atoms, or characterizedby having epoxy or alicyclic epoxy group at one or both sites ofbivalent group having, at a portion thereof, an aliphatic chain oralicyclic skeleton.R1-A1-R2   (11)R3-A2   (12)

In these general formula (11) or (12), R1, R2 and R3 representindividually epoxy group or epoxy group having an alicyclic skeleton; A1and A2 represent individually an aliphatic chain, an alicyclic skeleton,or a bivalent hydrocarbon group comprising an aliphatic skeleton or analicyclic skeleton each having 1 to about 15 carbon atoms. Among thecarbon atoms of the aliphatic chain or alicyclic skeleton, one or someof carbon atoms may be substituted by oxygen atom or constructed toprovide a ketone skeleton.

The compounds represented by the general formula (11) or (12) generallyhave a viscosity ranging from 5 mPa.sec to 100 mPa.sec, so that there islittle possibility that the inclusion of the compounds would badlyaffect the delivery of ink. In particular, although the inclusion ofalicyclic epoxy compounds is preferable in the case where theenhancement of acid-polymerizability or of the solubility ofphotosensitive agents is desired, it will tend to decrease the curingrate of ink and deteriorate the storage stability and solventresistance. Accordingly, an appropriate mixing ratio of these compoundsshould be confined within the range of 0 to 60% based on a total weightof the acid-curable compounds.

When the ink after curing is demanded to have plasticity and glossiness,the epoxy compounds represented by the following general formula (13)may be employed. The alicyclic epoxy compounds represented by thegeneral formula (13) generally have a viscosity of as high as 50 mPa.secto 10000 mPa.sec or more.R4-A3-(R5)_(k)   (13)

In the general formula (13), R4 and R5 represent individually epoxygroup or epoxy group having an alicyclic skeleton; and A3 represents(k+1)-valent functional group having at least alkylene group and/or analicyclic skeleton (k is a natural number).

For example, if a vinyl ether compound is incorporated into inkjet inkat a ratio ranging from 50 parts by weight to 90 parts by weight basedon 100 parts by weight of the inkjet ink, the aforementioned epoxycompound of high viscosity should preferably be incorporated into theinkjet ink at a ratio ranging from 10 parts by weight to 40 parts byweight based on 100 parts by weight of the inkjet ink. By doing so, itis possible to ensure minimum fluidity (a viscosity of 30 mPa.sec orless at a temperature of 50° C.) which is required in the delivery ofink. It is especially desirable that the weigh ratio between the vinylether compound and the high-viscosity compound is limited to the rangeof 1:1 to 10:1. If the vinyl ether compound and the high-viscositycompound are to be employed in combination as explained above, it isdesirable to employ the compounds represented by the aforementionedgeneral formula (4) or (5) as a photo-acid generating agent and to limitthe mixing ratio of the pigment to the range of 1 to 25 parts by weightbased on 100 parts by weight of the inkjet ink. In particular, when anepoxy compound of high molecular weight and having a viscosity of ashigh as 500 mPa.sec or more is incorporated into the ink, it would bepossible to obtain a printed layer excellent in plasticity andglossiness. According to the formulation as described above, it is nowpossible to prepare an inkjet ink which can be delivered or dischargedin an appropriate manner and is capable of providing a cured materialwith excellent plasticity and glossiness.

When the epoxy compounds represented by the following general formula(16) are employed in combination with the aforementioned vinyl ethercompounds, the storage stability of inkjet ink as well as the plasticityof ink layer can be enhanced.R¹¹—R¹²—(R¹¹)_(j)   (16)

In this general formula (16), R¹¹ is glycidyl ether group; R¹² isalkylene or hydroxyl-substituted alkylene group having 1 to 6 carbonatoms, or alkylene group having an alicyclic or hydroxyl-substitutedalicyclic skeleton and 6 to 15 carbon atoms; and j is an integer rangingfrom 1 to 3. Especially, diglycidyl ether compound of neopentyl glycolexhibits such an excellent properties that is almost unique in view ofviscosity, degree of acid polymerization, and storage stability.However, the addition of these compounds in an excessive quantity woulddeteriorate curing property of ink, so that the aforementioned epoxycompounds should preferably be incorporated at a ratio of 10 to 40 partsby weight based on 100 parts by weight of inkjet ink provided that themixing ratio of the vinyl ether compounds is confined within the rangeof 50 to 90 parts by weight based on 100 parts by weight of inkjet ink.

As for specific examples of the aforementioned epoxy compounds, theyinclude alicyclic epoxy compounds such as Celloxide 2021, Celloxide2021A, Celloxide 2021P, Celloxide 2081, Celloxide 2000 and Celloxide3000 (all available from Daicel Chemical Industries Ltd.);(metha)acrylate compounds having epoxy group, such as Cyclomer A200 andCyclomer M100; methacrylate having methylglycidyl group such as MGMA;glycidol representing a low molecular epoxy compound;β-methylepichlorohydrin; α-pinene oxide; α-olefin monoepoxide having 12to 14 carbon atoms; α-olefin monoepoxide having 16 to 18 carbon atoms;epoxidized soy bean oil such as Dimac S-300K; epoxidized linseed oilsuch as Dimac L-500; and polyfunctional epoxy compounds such as EpoleadGT301 and Epolead GT401.

It is also possible to employ alicyclic epoxy compounds (such asCylacure; Dow Chemical Co., Ltd, U.S.); low molecular weight phenolcompounds which are hydrogenated and aliphatized with terminal hydroxylgroup thereof being substituted by a group having epoxy; glycidyl etherof polyhydric aliphatic alcohol/alicyclic alcohol such as ethyleneglycol, glycerin, neopentyl alcohol, hexanediol and trimethylol propane;compounds having glycidyl ether group at an terminal of ethyleneoxide-based polymer; epoxy compounds wherein terpene compounds having anunsaturated bond are employed as a starting material and the unsaturatedbond is oxidized to obtain the epoxidized compounds; and glycidyl estersof hexahydrophthalic acid or hydrogenated aromatic polyhydric carboxylicacid.

(Oxetane Compounds)

Oxetane compounds can be suitably employed as a portion of theacid-polymerizable compounds. For example, in a situation where highspeed printing of several tens meters per minute is demanded or wherethe resistance to a solvent is required, it is preferable that anoxetane compound is incorporated in the ink. When oxetane compound isemployed as a major component in the conventional inkjet ink, theviscosity of the ink is generally caused to increase prominently, thusmaking it difficult to adjust the viscosity relative to other solvent.However, when the oxetane compound is employed in combination with avinyl ether compound, it can be easily formulated into ink since thevinyl ether compound is very low in viscosity.

As for the oxetane compound, it may be aromatic, aliphatic or alicyclicoxetane compound, and it may include ether bond in part of the structurethereof. With respect to mixing ratio between the oxetane compound andthe vinyl ether compound, it is preferable, in view of solventresistance, to employ an aromatic oxetane compound at a ratio rangingfrom 0 to 40% by weight, and it is also preferable, in view ofsuitability of viscosity, to employ the vinyl ether compound at a ratioof not less than 60% by weight. Further, in view of promoting the curingof ink, a total quantity of the oxetane compound in the ink shouldpreferably be at least 40% by weight or more, and furthermore, in viewof optimizing the cured hardness of ink, a total quantity of thecompounds having an alicyclic skeleton and an aromatic skeleton shouldpreferably be at least 30% by weight or more of the ink.

If the mixing ratios of these compounds fall outside these ranges, itmay become difficult to obtain an ink which is capable of satisfying allof the properties such as curing rate, delivery performance, solventresistance, etc.

Specific examples of aliphatic or alicyclic oxetane compounds of includedi[1-ethyl(3-oxetanyl)] methyl ether, 3-ethyl-3-(2-ethylhexyloxymethyl)oxetane, [(1-ethyl-3-oxetanyl)methoxy] cyclohexane,bis[(1-ethyl-3-oxetanyl)methoxy] cyclohexane or compounds comprising analicyclic group to which at least one oxetane-containing group isintroduced such as bis[(1-ethyl-3-oxetanyl)methoxy] norbornane. It isalso possible to employ ether compounds comprising aliphatic polyhydricalcohol such as ethylene glycol, propylene glycol and neopentyl alcohol,to which oxetane-containing alcohol such as 3-ethyl-3-hydroxymethyloxetane is dehydrocondensed.

Specific examples of the oxetane compounds comprising an aromaticskeleton include

-   -   1,4-bis[(1-ethyl-3 oxetanyl)methoxy] benzene,    -   1,3-bis[(1-ethyl-3 oxetanyl)methoxy] benzene,    -   4,4′-bis[(3-ethyl-3 oxetanyl)methoxy] biphenyl, and phenol        novolac oxetane.

However, the addition of an excessive quantity of these compounds maybring about malfunction in delivery of ink, so that the aforementionedoxetane compounds should preferably be incorporated at a ratio of 10 to60 parts by weight based on 100 parts by weight of inkjet ink providedthat the mixing ratio of the vinyl ether compounds is confined withinthe range of 50 to 90 parts by weight based on 100 parts by weight ofinkjet ink. In particular, when an oxetane compound of high molecularweight and having a viscosity of as high as 500 mPa.sec or more isincorporated into the ink, it would be possible to obtain a printedlayer excellent in plasticity and glossiness.

(Cyclic Esters, Cyclic Carbonate-Based Acid-Polymerizable Compounds)

Most of the photo-acid generating agents to be included as an essentialcomponent in the inkjet ink according the embodiments of the presentinvention are poor in solubility to acid-polymerizable compounds such asvinyl ether compounds. When the solubility of the photo-acid generatingagents to the inkjet ink is poor, the sensitivity of the ink wouldbecome insufficient on the occasion of irradiation of light, thuspossibly leading to an insufficient hardness of the cured film of ink.Otherwise, the photo-acid generating agent that has been failed todissolve may exist as an impurity such as coarse particles, thus raisingproblems such as the obstruction of stable delivery of ink.

In that case, it is effective to incorporate, into the ink, cycliccarbonate or cyclic lactone compounds which are capable of dissolvingthe photo-acid generating agents in a very excellent manner. Forexample, propylene carbonate which is one of cyclic carbonate has asolubility parameter of about 27 MPa^(1/2) and is liable to indicate ahigher polarity than that of the acid-polymerizable compounds.Therefore, even if a photo-acid generating agent which can be hardlydissolved in the acid-polymerizable compounds is to be employed, thesolubility of the photo-acid generating agent can be enhanced by theaddition of only a little amount of this propylene carbonate.

Further, propylene carbonate is polymeric, so that even if propylenecarbonate is employed in the inkjet ink, there is little possibilitythat propylene carbonate is discharged into air atmosphere as a volatilecomponent after the curing of ink layer. As for other specific examplesof cyclic carbonate or cyclic lactone compounds which can be preferablyemployed in all kinds of the inkjet ink according to the embodiments ofthe present invention, they include propylene carbonate propenylether,dimethyl carbonate, diphenyl carbonate, etc. However, since the additionof an excessive quantity of these compounds may deteriorate the curingproperty of ink, these cyclic compounds should preferably beincorporated at a ratio of 5 to 40 parts by weight based on 100 parts byweight of inkjet ink provided that the mixing ratio of the vinyl ethercompounds is confined within the range of 50 to 90 parts by weight basedon 100 parts by weight of inkjet ink.

(Other Kinds of Acid-Polymerizable Compounds that Can be Added to theInkjet Ink)

Additionally, the following compounds can be also added to the inkjetink.

Namely, it is possible to employ high-molecular weight compoundsincluding, for example, compounds having a molecular weight of 5000 orless and comprising a cyclic ether group such as epoxy group, oxetanegroup and oxorane group, which are bonded through a long chain alkylenegroup; acrylic or vinyl compounds having the aforementioned substituentgroups; carbonate compounds; low molecular melamine compounds; vinylethers; vinyl carbazoles; styrene derivatives; alpha-methylstyrenederivatives; vinyl alcohol esters including esters of vinyl alcohol withacryl or methacryl; monomers monomers having a cation polymerizablevinyl bond; and oligomers formed through the polymerization of at leastone kind of the monomers.

Further, the inkjet ink may contain the following compounds in additionto the aforementioned compounds. Namely, they include a homopolymer orcopolymer of vinyl alcohol; resins containing OH group, COOH group,acetal group, etc. and having a molecular weight of 5000 or less such ascasein and cellulose; polycarbonate resins having a molecular weight of5000 or less; copolymers to be derived from a reaction between polyamicacid, polyamino acid or acrylic acid and a vinyl compound having anacid-polymerizable double bond on its side chain; copolymers to bederived from a reaction between vinyl alcohol and a vinyl compoundhaving an acid-polymerizable double bond on its side chain; and methylolmelamine resin.

(Other Kinds of Compounds that Can be Incorporated into the Inkjet Ink)

The photosensitive compositions according to the present invention mayalso contain a basic additive, a dispersing agent, a surfactant, aradical polymeric monomer, a radical generating agent, a specific kindof solvent which is not acid-polymerizable, etc. Next, these componentswill be classified and explained in detail.

(Basic Compounds)

The inkjet ink should desirably be high in stability in inkjetdischarging. However, the inkjet ink is generally inclined to becomeprominent in the increase with time of viscosity so that the preferableproperties of them can be retained only a short period of time if theyare left as they are. Therefore, it is desirable that the inkjet inkadditionally contains, as a viscosity stabilizing agent, a basiccompound and/or a compound capable of expressing basicity. If carbonblack is employed as a color component, the effects of these viscositystabilizing agents can be exhibited more prominently. Moreover, sincethese basic compounds are concurrently capable of effectively protectingnot only the interior of the inkjet head of recording apparatus but alsothe metal portions of the piping for the ink from being eroded by acids,the employment of these basic compounds are preferable in all kinds ofthe inkjet ink according to the embodiments of the present invention.

As for the aforementioned basic compound, it is possible to employ anyinorganic basic materials as well as organic basic materials which arecapable of being dissolved in a compound that can be polymerized in thepresence of an acid. However, in view of solubility, the employment oforganic basic materials is more preferable. Specific examples of suchorganic basic materials include ammonia compound, ammonium compounds,substituted or unsubstituted alkylamine, substituted or unsubstitutedaromatic amine, pyridine, pyrimidine, and organic amines having ahetrocyclic skeleton such as imidazole. More specific examples of suchorganic basic materials include n-hexyl amine, dodecyl amine, aniline,dimethyl aniline, diphenyl amine, triphenyl amine, diazabicyclooctane,diazabicycloundecane, 3-phenyl pyridine, 4-phenyl pyridine, lutidine,2,6-di-t-butylpyridine, and sulfonyl hydrazides such as 4-methylbenzenesulfonyl hydrazide, 4,4′-oxybis(benzenesulfonyl hydrazide) and sulfonylhydrazide such as 1,3-benzenesulfonyl hydrazide.

Ammonium compounds can be also employed as a basic compound. Preferableexamples of the ammonium compounds are quaternary ammonium salts whichcan be represented by the following general formula (23).

(In this general formula (23), R_(a), R_(b), R_(c) and R_(d) may be thesame or different and are individually alkyl, cycloalkyl, alkylaryl oraryl, wherein one or more of aliphatic group CH₂ may be substituted byoxygen atom; and X3- represents a basic anion.)

Among the compounds represented by the aforementioned general formula(23), preferable examples thereof include the compounds wherein R_(a),R_(b), R_(c) and R_(d) are selected from the group consisting of methyl,ethyl, propyl, isopropyl, butyl, dodecyl, phenyl and benzyl; and X3- isselected from the group consisting of hydroxyl ion, ⁻OR (R is alkylhaving 1 to 4 carbon atoms), ⁻OCOR′(R′ is alkyl, aryl or alkylaryl),OCOO⁻ and OSOO⁻. Especially preferable examples of the basic compoundsare tetramethyl ammonium hydroxide and ammonium salt of tetrabutylhydroxide. These basic compounds can be employed singly or incombination of two or more.

With regard to the basicity of the basic compounds to be employed as aviscosity-stabilizing agent in the present invention, it is preferableto employ basic compounds exhibiting a base dissociation constant pKb of3 or more at a temperature of 25° C. and in a state of aqueous solutionthereof, provided that the vinyl ether compounds are employed singly orin combination with oxetane compounds. However, if the pKb of the basiccompounds is higher than 7, such compounds would be incapable ofexhibiting the effect of stabilizing the viscosity of ink. Suitableexamples of basic compounds which are capable of satisfying theaforementioned conditions are aliphatic amines and substituted aliphaticamines.

However, if the vinyl ether compounds of the present invention areemployed together with an epoxy compound and at the same time, a basiccompound exhibiting a very strong basicity such as imidazole is employedin this case, the polymerization thereof with time may occur orotherwise, a side reaction such as decomposition of photo-acidgenerating agent may be likely to take place. On the other hand, if abasic compound which is very weak in basicity is employed in this case,it would become difficult to sufficiently secure the effect ofstabilizing the viscosity of ink through the addition of the basiccompound. For example, it is preferable to employ basic compoundsexhibiting a base dissociation constant pKb of 4 or more at atemperature of 25° C. and in a state of aqueous solution thereof.However, if the pKb of the basic compounds is higher than 11, suchcompounds would be incapable of exhibiting the effect of stabilizing theviscosity of ink. Suitable examples of basic compounds which are capableof satisfying the aforementioned conditions are pyridine derivatives,aniline derivatives, aminonaphthalene derivatives, other kinds ofnitrogen-containing heterocyclic compounds and the derivatives thereof.

Specific examples of the pyridine derivatives include 2-fluoropyridine,3-fluoropyridine, 2-chloropyridine, 3-chloropyridine, 3-phenylpyridine,2-benzylpyridine, 2-formylpyridine, 2-(2′-pyridyl) pyridine,3-acetylpyridine, 2-bromopyridine, 3-bromopyridine, 2-iodopyridine,3-iodopyridine, and 2,6-di-tert-butylpyridine.

Specific examples of the aniline derivatives include aniline,4-(p-aminobenzoyl) aniline, 4-benzylaniline,4-chloro-N,N-dimethylaniline, 3-5-dibromoaniline, 2,4-dichloroaniline,N,N-dimethylaniline, N,N-dimethyl-3-nitroaniline, N-ethylaniline,2-fluoroaniline, 3-fluoroaniline, 4-fluoroaniline, 2-iodoaniline,N-methylaniline, 4-methylthioaniline, 2-bromoaniline, 3-bromoaniline,4-bromoaniline, 4-bromo-N,N-dimethylaniline, 2-chloroaniline,3-chloroaniline, 4-chloroaniline, 3-chloro-N,N-dimethylaniniline,3-nitroaniline, 4-nitroaniline, 2-methoxyaniline, 3-methoxyaniline,diphenylamine, 2-biphenylamine, o-toluidine, m-toluidine, p-toluidine,3,3′-diaminodiphenyl sulfone, 4,4′-diaminodiphenyl sulfone, and4,4′-bis(4-aminophenoxy) diphenyl sulfone.

Specific examples of the aminonaphthalene derivatives include, forexample, 1-amino-6-hydroxynaphthalene, 1-naphthylamine, 2-naphthylamine,diethylaminonaphthalene, and N-methyl-1-naphthylamine.

Specific examples of other kinds of nitrogen-containing heterocycliccompounds and the derivatives thereof include, for example, cinnoline,3-acetylpiperidine, pyrazine, 2-methylpyraxzine, methylaminopyrazine,pyridazine, 2-aminopyrimidine, 2-amino-4,6-dimethylpyrimidine,2-amino-5-nitropyrimidine, 2,4,6-triamino-1,3,5-triazine, pyrrol,pyrazole, 1-methylpyrazole, 1,2,4-triazole, indazole, benzotriazole,quinazoline, quinoline, 3-aminoquinoline, 3-bromoquinoline,8-carboxyquinoline, 3-hydroxyquinoline, 6-methoxyquinoline,5-methylquinoline, quinoxaline, thiazole, 2-aminothiazole,3,4-diazaindole, purine, 8-azapurine, indole and indolizine.

As for the aliphatic amines and the derivatives thereof, aminederivatives represented by the following general formula (21) can beemployed.

In the general formula (21), R²¹, R²² and R²³ may be the same ordifferent and at least one of them is substituted or unsubstituted alkylgroup and the rest are individually hydrogen atom, hydroxyl group orsubstituted or unsubstituted alkyl group. The amine compoundsrepresented by the general formula (21) can be used in combination withoxetane compounds or vinyl ether compounds.

When the aforementioned basic compounds are capable of forming a salt incombination with anions represented by the following general formula(22) and the anions are low in acidicity, the basic compounds would actby itself as a weak base. Therefore, the salts can be employed in thesame manner as described above.

In the general formula (22), A⁻ is a compound having sulfonate anion orcarboxyl anion; R²¹, R²² and R²³ may be the same or different and areindividually substituted or unsubstituted aromatic group, hydrogen atom,hydroxyl group or substituted or unsubstituted alkyl group.

The inkjet ink according to the embodiments of the present invention maybe heated after a step of exposure, so that the volatility of the basiccompounds should preferably be as low as possible. More specifically,the boiling point of the basic compounds should preferably be 150° C. ormore, more preferably 180° C. or more at the ordinary pressure.

The basic compounds or compounds capable of exhibiting basicity, bothbeing employed as a viscosity-stabilizing agent, should preferably beincorporated into the inkjet ink at a ratio of 1 to 30 mol %, morepreferably at a ratio of 2 to 15 mol % based on a total molar quantityof a photo-acid generating agent. If mixing ratio of these compoundsfalls outside this range, the sensitivity of ink would be considerablydeteriorated or the effects thereof to stabilize the viscosity of inkwould be varnished.

When a photosensitive basic compound that can be decomposed through theirradiation of light or radiation is employed, any decrease ofsensitivity due to the addition of a basic compound can be minimized andhence the employment of such a photosensitive basic compound isreferable.

As for the photo-sensitive basic compound, it is possible to preferablyemploy sulfonium compounds and iodonium compounds. As for the sulfoniumcompounds, the compounds represented by the following general formulas(SS1) to (SS4) can be exemplified.

(Wherein R³¹, R³² and R³³ may be the same or different and areindividually alkyl group, aryl group, heteroaryl group, or aryl groupsubstituted by alkyl group, alkylaryl group, halogen atom, alkoxy group,phenoxy group, thiophenol group, phenylsulfonyl group or phenylsulfenylgroup; Y is CH₂, O or S; R³⁴, R³⁵, R³⁶ and R³⁷ may be the same ordifferent and are individually alkyl group, alkoxy group or halogenatom; and X1⁻ represents a basic anion.)

Preferable examples of R³¹, R³² and R³³ include methyl, ethyl, propyl,isopropyl, butyl, phenyl, biphenyl, tolyl, xylyl, chlorophenyl,bromophenyl, methoxyphenyl, ethoxyphenyl, propyloxyphenyl,butyloxyphenyl, tert-butyloxyphenyl, phenoxyphenyl, thiophenoxyphenyl,phenoxyphenyl, thiophenoxyphenyl and phenylsulfonylphenyl.

Preferable examples of R³⁴, R³⁵, R³⁶ and R³⁷ include alkyl group,methoxy group, ethoxy group, chlorine atom and bromine atom.

Specific examples of X1⁻ include anions such as hydroxyl ion, ⁻OR (R isalkyl having 1 to 4 carbon atoms), ⁻OCOR′ (R′ is alkyl, aryl oralkylaryl), OCOO⁻ and OSOO⁻.

As for the iodonium compounds, the compounds represented by thefollowing general formulas (IS1) to (IS3) can be exemplified.

-   -   wherein R³⁸ and R³⁹ may be the same or different and are        individually alkyl group, aryl group, heteroaryl group or mono,        di or tri-substituted aryl group which is substituted by alkyl        group, aryl group, halogen atom, alkoxy group, phenoxy group,        thiophenol group, phenylsulfonyl group or phenylsulfenyl group;        Y is CH₂, O or S; R⁴⁰, R⁴¹, R⁴² and R⁴³ may be the same or        different and are individually alkyl group, alkoxy group or        halogen atom; n is an integer of 5 or 6; and X2⁻ represents a        basic anion.

Preferable examples of R³⁸ and R³⁹ include methyl, ethyl, propyl,isopropyl, butyl, phenyl, biphenyl, tolyl, xylyl, chlorophenyl,bromophenyl, methoxyphenyl, ethoxyphenyl, propyloxyphenyl,butyloxyphenyl, tert-butyloxyphenyl, phenoxyphenyl, thiophenoxyphenyl,phenoxyphenyl, thiophenoxyphenyl and phenylsulfonylphenyl.

Preferable examples of R⁴⁰, R⁴¹, R⁴² and R⁴³ include alkyl group,methoxy group, ethoxy group, chlorine atom and bromine atom.

Specific examples of X2- include anions such as hydroxyl ion, ⁻OR (R isalkyl having 1 to 4 carbon atoms), ⁻OCOR′ (R′ is alkyl, aryl oralkylaryl), OCOO⁻ and OSOO⁻.

Especially preferable examples of such sulfonium compounds and iodoniumcompounds are triphenylsulfonium acetate, triphenyl sulfonium hydroxide,triphenyl sulfonium phenolate, tris-(4-methylphenyl) sulfoniumhydroxide, tris-(4-methylphenyl) sulfonium acetate,tris-(4-methylphenyl) sulfonium phenolate, diphenyl iodonium hydroxide,diphenyl iodonium acetate, diphenyl iodonium phenolate,bis-(4-t-butylphenyl) iodonium hydroxide, bis-(4-t-butylphenyl) iodoniumacetate, bis-(4-t-butylphenyl) iodonium phenolate,thiophenyl-substituted triphenylsulfonium acetate, andthiophenyl-substituted triphenylsulfonium hydroxide.

In addition to the aforementioned basic compounds, it is also possibleto incorporate other basic compounds into the ink. Further, if oniumsalts are to be employed as a photo-acid generating agent, thephoto-acid generating agent should preferably be selected from thosewhich are similar in kinds to the basic compound. For example, if thephoto-acid generating agent and the basic compound are both formed ofsulfonium compounds or iodonium compounds, it would be possible toobtain excellent effects in terms of sensitivity and shelf lifestability.

Alternatively, it is also possible to preferably employ a compound whichis not inherently basic but is capable of generating a basic compoundthrough the decomposition thereof with time, since the basic compoundthus generated can act to neutralize acids that may be allowed togradually generate. Specific examples of such a compound includecompounds which are capable of generating base through heating. Forexample, it is possible to employ NBC-101 (trade name; Midori KagakuCo., Ltd.), carbamate compounds such as α,α-dimethyl-3,5-dimethoxybenzylcarbamate.

It is also possible to preferably employ the compounds represented bythe following general formulas (TBG1) to (TBG8) can be employed.

-   -   wherein R⁵¹, R⁵² and R⁵⁴ may be the same or different and are        individually hydrogen atom, linear or branched alkyl group        having 1 to 20 carbon atoms, or cyclic alkyl group having 3 to        20 carbon atoms, wherein a pair of R⁵¹ and R⁵², a pair of R⁵¹        and R⁵⁴ and a pair of R⁵² and R⁵⁴ may be combined with each        other to form a cyclic structure; R⁵³ is linear or branched        alkyl group having 1 to 4 carbon atoms; R⁵⁵ is hydrogen atom,        linear or branched alkyl group having 1 to 20 carbon atoms,        cyclic alkyl group having 3 to 20 carbon atoms, or aryl group        having 6 to 20 carbon atoms; M is iodonium or sulfonium; j is an        integer ranging from 2 to 10; k is an integer ranging from 1 to        3.

As for the linear or branched alkyl group having 1 to 20 carbon atoms,which can be introduced as R⁵¹, R⁵², R⁵⁴ and R⁵⁵, they may be selectedfrom methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl andn-dodecyl. As for the cyclic alkyl group having 3 to 20 carbon atoms, itis possible to employ, for example, cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl or cyclododecyl. As forthe aryl group having 6 to 20 carbon atoms, it is possible to employphenyl, naphthyl or pyrenyl. As for the cyclic structure to be formedfrom the aforementioned pair of groups, it includes piperidino group,pyrrolidino group, propylene imino group and azetidino group.

As for the linear or branched alkyl group having 1 to 4 carbon atoms,which can be introduced as R⁵³, it may be selected from methyl, ethyl,n-propyl, isopropyl, n-butyl, tert-butyl, etc.

(Dispersing Agent, Surfactant, etc.)

In order to further enhance the dispersibility of pigment, etc., theinkjet ink according to the embodiments of the present invention mayinclude a small quantity of dispersing agent such as a nonionic or ionicsurfactant and an antistatic agent. High-molecular dispersing agentshaving almost the same properties as the aforementioned dispersingagents, such as acrylic polymer and polyvinyl alcohol, can be alsosuitably employed. However, when a cationic dispersing agent is employedas a dispersing agent, it is preferable to select a compound whoseacidity is lower than that of carboxylic acid. The reason is that somekinds of cationic dispersing agents may promote the curing dark reactionof the ink. Further, those kinds of dispersing agents and dyes thatexhibit strong basicity are undesirable as they not only deteriorate thesensitivity of ink but also promote the curing dark reactionoccasionally. Therefore, the dispersing agents should preferably beselected from those exhibiting nearly neutrality or from nonionicdispersing agents.

(Radical Polymeric Monomers)

When a printing surface is highly basic, or when a pigment or a printingsurface is liable to be affected by an acid, the influence of the acidcan be minimized by the incorporation of a suitable radicallypolymerizable compound. As for the radically polymerizable compound,they include, for example, an acrylic monomer, a methacrylic monomer, astyrene monomer, and a compound having a plurality of vinyl-basedpolymerizable groups of these monomers.

The vinyl ether compound can be radically polymerizable singly or incombination with an acrylic monomer. Likewise, when a compound providedwith radically polymerizable as well as cationically polymerizableproperties, such as CEL2000 (trademark, Daicel Chemical Industries.,Ltd.), glycidyl methacrylate, and ester compounds to be derived fromvinyl alcohols and acrylic acid, methacrylic acid, etc. is added to theink, it would be possible to obtain advantages associated with bothradical polymerizability and cationic polymerizability. In this case., aphotoradical polymerization initiator such as Michler's ketone known asIrgar Cure (trade mark) and benzophenone, or a photocrosslinking typeradical generating agent such as bisazide can be incorporated into theink together with the aforementioned radically polymerizable compound.This technique may be employed also in a case where the ink layer isrequired to have excellent chemical resistance after the curing thereof.

(Other Kinds of Solvent)

The liquid ink according to the embodiments of the present inventionshould preferably be prepared so as not to include any volatilecomponent such as water and organic solvents in general. However,organic solvents which are to be employed in the preparation of rawmaterials, such as methylethyl ketone, propylene glycol-based solvents,ethyl lactate and xylene may be contained therein in an unavoidablequantity. Further, if the inkjet recording apparatus is provided with agas exhausting mechanism or a solvent recovery mechanism for instance, alittle quantity of organic solvents may be contained in the solution forthe purpose of obtaining desired printed matters. In this case, it ispreferable, in safety viewpoint, to employ water, alcohols such asethanol, propanol or petroleum components such as isoper and terpene.

In the foregoing explanations described above, the present invention hasbeen explained in detail by referring to the specific examples of eachcomponent. Next, characteristics which are required commonly in all ofinkjet inks will be explained.

When the alicyclic skeleton included in the aforementionedacid-polymerizable compounds is formed of terpenoid skeleton, it wouldpreferably enhance the safety of inkjet ink or the ink layer after thecuring thereof to human body as well as to the environments. Examples ofsuch a structure include vinyl ether compounds wherein hydrogen atom ofalcohols having the aforementioned skeleton is substituted by vinylgroup, the vinyl ether compounds being formed of derivatives such asmyrcene, ocimene, geraniol, nerol, linalol, citrorenol, citral,menthene, limonene, dipentene, terpinolene, terpinene, phellandrene,sylvestrene, piperithol, terpineol, menthenemonol, isopregol, peraryaldehyde, piperitone, dihydrocarvone, carvone, pinol, ascaridole,sabinene, carene, pimene, bornene, fenchene, camphene, carveol,sesquiterpene, diterpene, triperpene, etc.

Alternatively, it is also possible to preferably employ alicyclicoxetane compounds wherein alcohols having the aforementioned skeleton isether-linked, through dehydrocondensation, with the alcohols having anoxetane skeleton. On the other hand, vinyl ether compounds, epoxycompounds and oxetane compounds all having a norbornene skeleton, whichcan be abundantly found in natural environment, are advantageous interms of cost and hence can be preferably employed.

Since the inkjet ink according to the embodiments of the presentinvention is a photosensitive ink which requires the application ofheating, the volatility of ink should preferably be as low as possiblein viewpoint of safety and odor. More specifically, the volatilizingrate of ink after the exposure thereof and at a temperature of 80° C.should preferably be confined to not higher than 0.2 mg/cm^(2.)min. Thequantity of volatilization herein is a quantity of volatilization (mg)per minute that will be generated when a vessel having an opening areaof 10 cm² for instance is heated. Although this quantity ofvolatilization varies depending on the size of the opening of thevessel, this value is usually defined as a value that can be obtainedwhen a Petri dish having a diameter of 6 cm and containing 4 g of inktherein is heated under the ordinary pressure. When the rate ofvolatilization becomes too large, the environmental safety would bedeteriorated and at the same time, the problem of odor would become moresevere. On the other hand, if the ink is formulated such that thevolatility thereof is very poor, e.g. 0.00001 mg/cm^(2.)min or less, theviscosity of the ink would become too high in general so that there ismuch possibility that the delivery of inkjet would become difficult.

As already explained, the inkjet ink according to the embodiments of thepresent invention should preferably be formulated so as to have afluidity of at least 30 mPa.s at the ordinary temperatures. In order toenable the ink to have the aforementioned properties and when the inkcontains “n” kinds of acid-polymerizable compounds including theaforementioned vinyl ether compounds, the viscosity η_(t) represented bythe following formula (A) should preferably be confined within the rangeof 3 (mPa.s) to 30 (mPa.s), more preferably 5 (mPa.s) to 25 (mPa.s).$\begin{matrix}\begin{matrix}{\eta_{t} = {\exp( {{\chi_{1} \cdot {\ln( \eta_{1} )}} + {\chi_{2} \cdot {\ln( \eta_{2} )}} +} }} \\ {{\chi_{3} \cdot {\ln( \eta_{3} )}} + \ldots + {\chi_{n} \cdot {\ln( \eta_{n} )}}} )\end{matrix} & (A)\end{matrix}$

-   -   (wherein χ₁, χ₂, χ₃ . . . χ_(n) represent weight ratios of each        of the components; and η₁, η₂, η₃ . . . η_(n) represent the        viscosity of the individual component at the ordinary        temperature and the ordinary pressure)

If the viscosity η_(t) is failed to fall within this range, the deliveryof ink may become very difficult or disturbance of image may be causedto occur due to irregular delivery of ink. Further, when the value ofthis η_(t) is 12 mPa.s or less, the inkjet ink can be delivered withoutnecessitating the heating of the head member, thus making the ink morepreferable in the storage thereof or in minimizing the evaporationthereof from the head member.

Further, the inkjet ink according to the embodiments of the presentinvention may include, as part of the solvent, an epoxy compound or anoxetane compound having a solubility parameter of 19 MPa^(1/2) or more.When these compounds are included in the inkjet ink, the propertiesrelated to the solubility of ink can be controlled to further enhancethe stability of the ink. For example, by the inclusion of thesecompounds, it is now possible to obtain various effects such as theenhancement of solubility of a photo-acid generating agent, theenhancement of dispersibility of pigment employed as a coloringmaterial, and still more, the enhancement of surface features of theink. If the surface tension of ink is not appropriate under theconditions where the action of delivering the ink is affected such assurface tension, it has been required to incorporate a surfactant in theink to adjust the value of surface tension. However, when a compoundhaving a predetermined solubility parameter is incorporated in the ink,the addition of such a surfactant would not be required any longer.

Vinyl ether compounds can be easily dissolved in an acid-polymerizablecompound having a solubility parameter of 19 MPa^(1/2) or more, therebymaking it possible to optionally set the solubility parameter of ink toa range which enables a stable delivery action of inkjet ink. Thissolubility parameter can be determined from the solubility to a materialwhose evaporation heat, surface tension and solubility parameter arealready known. Specific examples of epoxy compounds and oxetanecompounds useful in this case include C3000 (limonene dioxide; Daicelchemical Industries, Ltd.), OXT-221 (di[1-ethyl (3-oxetanyl)]methylether; Toa Gosei Co., Ltd.) and a compound having polyethylene oxideskeleton and oxetane group at a terminal thereof.

As already explained above, the image-forming capability of the inkjetink according to the embodiments of the present invention dependslargely on the chemical amplification mechanism. Namely, an acidgenerates from a photo-acid generating agent due to the exposurethereof, and the acid thus generated is diffused due to the heatingthereof, thus enabling the acid to function as a catalyst for thecrosslinking reaction. Therefore, in the case of the inkjet ink, thepresence of basic ions in a great magnitude would become a cause fordeteriorating the sensitivity of the ink. Therefore, attention should bepaid so that the inkjet ink can be prevented from being contaminated bya large quantity of basic ions not only in the process of preparing theinkjet ink but also even in the process of manufacturing each of theconstituent components.

The pigments to be employed in the inkjet ink according to theembodiments of the present invention may be further provided with otherproperties such as magnetism, fluorescence, conductivity, dielectricproperty, electromagnetic exothermic property, etc. in addition to thecoloring and tinting properties. If the inkjet ink containing such apigment provided with these various properties is employed, it maybecome possible to obtain a printed matter having various functions.Next, some of such examples will be explained.

At first, a case where the inkjet ink contain a pigment provided withmagnetism will be explained.

First of all, an inkjet ink containing, as a pigment, powder providedwith magnetism is prepared. This inkjet ink can be obtained byincorporating magnetic powder made of, for example, iron, cobalt,nickel, an alloy or oxide thereof into any of the aforementionedsolvents together with a photo-acid generating agent and ahigh-molecular compound or powder which is capable of enhancing thedispersibility of ink.

Then, this inkjet ink is delivered onto a recording medium to draw apattern such as a bar code pattern. After finishing the delivery of ink,light such as ultraviolet ray is immediately irradiated against the inklayer. The quantity of irradiation on this occasion may be determineddepending on the mixing ratio of the pigment in the inkjet ink or on thesensitivity of the inkjet ink, the dosage of the irradiation beingordinary such that ranges from several hundreds mJ to one thousand andseveral hundreds mJ. Even immediately after the irradiation of light,the stickiness and fluidity of the ink layer would be varnished.However, the ink layer may be left to stand in a stocker at atemperature ranging from the ordinary temperature to 60° C., thuscompletely curing the ink layer. Since the ink layer cured in thismanner is provided with magnetism, it is possible to read out asecondary information other than image information by a magnetismdetecting mechanism such as a magnetic head.

Next, a case where the inkjet ink contains a conductive pigment will beexplained.

First of all, an inkjet ink containing, as a pigment, powder providedwith electric conductivity is prepared. This inkjet ink can be obtainedby incorporating a conductive pigment selected from, for example,silver, gold, copper, aluminum, carbon, nickel, iron, cobalt, lead, tin,antimony, an alloy comprising an optional combination of these metalsand a composite thereof with an organic material into any of theaforementioned solvents together with a photo-acid generating agent anda high-molecular compound or powder which is capable of enhancing thedispersibility of ink. Alternatively, for the purpose of reducing thecontent of resin component so as to enhance the electric conductivity ofthe ink layer that can be ultimately obtained, the mixing ratio of thepolymeric compounds in the inkjet ink may be reduced and at the sametime, an organic solvent may be incorporated into the inkjet ink.

Then, this inkjet ink is delivered onto a recording medium such as aninsulating substrate to draw a pattern such as a wiring pattern. Afterfinishing the delivery of ink, light such as ultraviolet ray isimmediately irradiated against the ink layer. The quantity ofirradiation on this occasion may be determined depending on the mixingratio of the pigment in the inkjet ink or on the sensitivity of theinkjet ink, the dosage of the irradiation being ordinary such thatranges from several hundreds mJ to one thousand and several hundreds mJ.Even immediately after the irradiation of light, the stickiness andfluidity of the ink layer would be varnished. However, the ink layer maybe left to stand in a stocker at a temperature ranging from the ordinarytemperature to 60° C., thus completely curing the ink layer. Theelectric conductivity of the ink layer can be further enhanced byapplying heat and pressure to the ink layer. Further, when alloy powderwhich is relatively low in melting point is employed as a conductivepigment, the ink layer may be heated up to a fluidizing temperature ofthe ink to obtain a conductive pattern. Since the printed patternobtained in this manner is provided with electric conductivity, it ispossible to employ the pattern as a circuit pattern or a resistorpattern.

Alternatively, when an inkjet ink containing, as a pigment, dielectricpowder made of an alloy or oxide of barium, lead, bismuth, iridium,ruthenium, tantalum, platinum, titanium, strontium or chromium; orceramic power is employed, it is possible to form a dielectric layer ofcapacitor or inductor. In this case, when the ink layer is sinteredafter it is formed into a pattern, it may become possible to enhance theproperty thereof. Further, when an inkjet ink containing a pigmentprovided with photocatalizing function or bactericidal action such astitanium oxide powder is employed, it is possible to form a printedpattern having such a function and action. As for the electromagneticexothermic powder, it is possible to employ, for example,electromagnetic exothermic ceramics or silicone resin. Thiselectromagnetic exothermic powder can be employed for the purpose ofselectively heating a printed portion by electromagnetic wave. Thispowder can be employed for enhancing the electromagnetic heatingproperty of all kinds of the inkjet ink according to the embodiments ofthe present invention.

Furthermore, by taking advantage of the techniques illustrated in theseembodiments, it is possible to perform overlapped printing or to form arelatively thick printed pattern. More specifically, a step ofdelivering inkjet ink onto a predetermined region of a recording mediumto obtain an ink layer and a step of curing the ink layer are repeated aplurality of times, thereby making it possible to form an image whichhas been partially corrected or an image having an embossed portionwhich can be tactilely recognized. For example, it is possible to form aprinted pattern for the handicapped such as Braille, a printed patternsuch as a topographic map where the thickness thereof is fluctuated inconformity with the contour lines, and a portion of device having athickness of several tens μm or more.

The inkjet ink according the embodiments of the present invention can beapplied to an inkjet recording apparatus shown in FIG. 1 for instance.The inkjet recording apparatus 1 shown in FIG. 1 is provided with atransferring mechanism 3 for transferring a recording medium 2. Alongthe running direction of the transferring mechanism 3, there aresuccessively disposed, starting from the upstream side to the downstreamside, an inkjet type recording head 4, a light source 5 and a heater 6as a heating mechanism.

With respect to the recording medium (an printing matter or an articleto which printing is applied) 2, there is not any particular limitationon the kinds thereof as long as printing can be performed on therecording medium 2. Namely, it is possible to employ, as the recordingmedium 2, various kinds of materials including paper, an OHP sheet, aresin film, non-woven fabric, a porous film, a plastic plate, a circuitboard and a metallic substrate.

The transferring mechanism 3 is constructed so as to enable therecording medium 2 to pass through the recording head 4, the lightsource 5 and the front side of heater 6. In this case, the transferringmechanism 3 transfers the recording medium 2 from the right side to theleft side in the drawing. This transferring mechanism 3 can beconstituted for example by a belt and/or a roller for transferring therecording medium 2, and a driving mechanism for driving the belt and/orthe roller. Further, this transferring mechanism 3 may be furtherprovided with a guiding member for assisting the transfer of therecording medium 2.

The recording head 4 is configured so as to deliver an inkjet ink ontothe recording medium 2 according to image signals, thereby forming anink layer. As for the recording head 4, it is possible to employ aserial scanning type head mounted on a carriage or a line scanning typehead having a width larger than that of the recording head 4. Inviewpoint of achieving a high-speed printing, the latter is generallymore advantageous as compared with the former. With respect to themethod of delivering an inkjet ink from the recording head 4, there isnot any particular limitation. For example, by using the pressure ofvapor to be generated by the heat of a heating element, the droplets ofink can be ejected. Alternatively, by using a mechanical pressure pulseto be generated from a piezoelectric element, the droplets of ink can beejected.

The light source 5 irradiates light to the ink layer formed on therecording medium 2, thus enabling acid to generate from the ink layer.As for the light source 5, it is possible to employ a mercury lamp suchas a low, medium or high pressure mercury lamp; a tungsten lamp; an arclamp; an excimer lamp; an excimer laser, a semiconductor laser; a YAGlaser; a laser system constituted by a combination of laser andnon-linear optical crystal; a high-frequency induction type ultravioletsgenerating apparatus; an electron beam irradiating apparatus; an X-rayirradiating apparatus; etc. Among them, the employment of thehigh-frequency induction type ultraviolets generating apparatus and thehigh/low pressure mercury lamp or the semiconductor laser would be morepreferable, since these devices are advantageous in simplifying thesystem involved. The light source 5 may be provided with a convergingmirror or a sweep optical system.

The heater 6 employed as a heating mechanism heat the ink layer formedon the recording medium 2, thus promoting the crosslinking reactionusing an acid as a catalyst. Although it is possible to enable theinkjet ink according to the embodiments of the present invention to curewithout necessitating the application of heat after the exposurethereof, it is desirable perform the heating of the ink for enabling theink to cure more quickly. As for this heater 6, it is possible to employan infrared lamp; a roller (heat roller) housing therein a heatingelement; a blower for ejecting hot air or heated air; etc.

By using the aforementioned apparatus 1, the printing to the recordingmedium can be performed by the following method.

First of all, by the transferring mechanism 3, the recording medium 2 istransferred from the right side to the left side in the drawing. Thetransferring speed of the recording medium 2 may be confined for examplewithin the range of 0.1 m/min to 100 m/min.

As the recording medium 2 is transferred up to a location in front ofthe recording head 4, the aforementioned inkjet ink is delivered fromthe recording head 4 according to the image signals. As a result, aprescribed ink layer (not shown) is formed on the recording medium 2.

Then, the recording medium 2 having the ink layer formed thereon istransferred to a location in front of the light source 5. At the momentwhen the recording medium 2 passes through a location in front of thelight source 5, light is irradiated from the light source 5 to the inklayer formed on the recording medium 2, thereby enabling an acid togenerate in the ink layer. Incidentally, the intensity of irradiatedbeam at the surface of ink layer may be confined generally within therange of several mW/cm² to 1 KW/cm² though this range may be varieddepending on the wavelength of the light source to be employed. Thequantity of exposure to the ink layer can be optionally determineddepending on the sensitivity of inkjet ink or on the transferring speedof the printing matter (i.e. the transferring speed of the recordingmedium 2).

Thereafter, the recording medium 2 is transferred to the interior of orin the vicinity of the heater 6. At the moment when the recording medium2 passes through the interior of or in the vicinity of the heater 6, theink layer formed on the recording medium 2 is heated by the heater 6 topromote the crosslinking reaction in the ink layer. Incidentally, in theapparatus shown in FIG. 1, the heating time by the heater 6 is generallyrelatively short, i.e. ranging from several seconds to several tensseconds. Accordingly, if it is desired that the curing of the ink layeris substantially completely accomplished by the heater 6, the heatingshould be performed with the maximum ultimate temperature beingcontrolled to become relatively high, i.e. 200° C. or less, morepreferably within the range of 80° C. to 200° C., most preferably withinthe range of about 60° C. to 180° C.

Subsequently, the recording medium 2 is transferred to the interior ofstocker (or vessel) (not shown), thus finishing the printing.

The heating means for heating the ink layer may not be restricted to theheater 6 which is disposed at a downstream side of the light source asshown in FIG. 1. For example, the light source S may be utilized as aheating source by moving the light source 5 close to recording medium 2on the occasion of irradiating light to the ink layer while taking careso as not to damage the printing surface. Likewise, it is also possibleto utilize the light source as a heating source by omitting theprovision of a heat-dissipating mechanism such as a cold mirror. When ahigh-output bulb of several hundreds watts is employed as a lightsource, a cooling mechanism which must be provided together with thelight source can be modified so as to intentionally return the wasteheat to the surface of printing paper. More specifically, part of thewaste heat mechanism of the cooling mechanism is modified so as toprovide a mechanism which makes it possible to return the waste heat tothe printing paper as described above. By the provision of such amechanism, the ink layer can be heated by using heat to be generatedfrom the light source.

For example, by reintroducing an air flow which has been utilized forcooling the light source onto the surface of printing paper or into theinterior of transferring/sustaining mechanism, it will be possible toobtain a mechanism which corresponds to a light source having an outputof not less than one hundred watts and provided with a heatingmechanism. The ultimate temperature of the recording medium that can berealized through the recirculation of heat of the light source may besuch that makes it possible to obtain the same degree of effect asobtainable from the heating by the aforementioned heater. Although itdepends on the heating time, a preferable range of temperature may begenerally at least 60° C. or more, more preferably within the range of80° C. to 100° C. When the speed of exposure is relatively high, e.g.several meters/sec., the ultimate temperature may be as high as 180° C.or so since the recording medium will be heated instantaneously.

When a light source which is capable of generating infrared ray inaddition to the visible light is employed as the light source 5 forinstance, the irradiation of light and the heating to the recordingmedium can be concurrently performed. The employment of such a lightsource is preferable since-it will promote the curing of ink layer.

Since the ink layer is heated also from the heat to be generated fromthe light source 5 as the ink layer is irradiated with light, theheating mechanism may not necessarily be installed as an independentmember as in the case of the heater 6. However, if only the heat fromthe light source 5 is utilized for heating the ink layer and then theheated ink layer is left to stand at the ordinary temperature forcompletely curing the ink layer, it will take a long time. Therefore,the standing of ink layer at the ordinary temperature should preferablybe applied to when sufficiently a long period of time can be secured forcomplete curing of the ink layer. For example, in the case of theprinted matters such as newspaper which will be distributed next day, itis possible to secure such a long time as about a whole day and nightfor allowing the ink layer to cure, and therefore it is possible tocompletely cure the ink layer even if the ink layer is left to stand atthe ordinary temperature.

The inkjet ink according to the embodiments of the present invention canbe also applied to the inkjet recording apparatus shown in FIG. 2. Theinkjet recording apparatus 1 shown in FIG. 2 is constructed in the samemanner as the inkjet recording apparatus shown in FIG. 1 except that thestocker 7 is provided therein with a heater.

The stocker 7 accommodates a plurality of recording medium 2 that havebeen finished the exposure. This stocker 7 is capable of keeping therecording medium 2 after exposure at a temperature of about 40° C. ormore, preferably within the range of about 50° C. to 120° C., morepreferably within the range of about 60° C. to 100° C. or about 50° C.to 80° C. for a predetermined period of time.

In the case of the apparatus 1 shown in FIG. 1, since the heater 6 isdisposed in a midway of the transferring route of the recording medium2, it would not be easy to heat individual recording medium 2 for a longperiod of time. Whereas, in the case of the apparatus 1 shown in FIG. 2,since the heater is disposed inside the stocker 7, it is possible toheat individual recording medium 2 for a long period of time. Moreover,since a plurality of recording medium 2 can be heated collectively inthe interior of the stocker 7, it would be advantageous in viewpoint ofsaving the power consumption. Furthermore, since individual recordingmedium 2 can be heated for a long period of time, it is now possible toobtain the following advantages.

Namely, depending on the combination between the recording medium 2 andthe inkjet ink to be employed, refluidization may take place as therecording medium 2 is heated at a high temperature for a short period oftime, thereby possibly deteriorating the quality of the printed image.Since this deterioration in quality of the printed image is caused dueto the refluidization at a high temperature, this deterioration inquality of the printed image can be prevented by performing the heatingof the ink layer at a relatively low temperature for a long period oftime. For example, when the recording medium 2 after the exposurethereof is permitted to stand inside the stocker 7 for several minutesat a temperature ranging from about 50° C. to 80° C., the ink layer canbe substantially completely cured without inviting the deterioration inquality of the printed image.

Preferably, the stocker 7 should be provided with a dischargingmechanism for allowing the recording medium 2 that has been kept in aheated state for a predetermined period of time to successively move outof the stocker 7. The retention time for heating can be determineddepending on the kinds of inkjet ink to be employed as well as on theheating temperature. For example, when the heating temperature is set tothe range of 50° C. to 100° C., the retention time for heating may beset to the range of about 0.5 minute to 10 minutes. When the heating isperformed without heater, the retention time for heating may be severalhours.

The heater 6 shown in FIG. 1 may be additionally disposed on theupstream side of the stocker 7. If the degree of curing of the ink layeris not sufficient enough in the process of transferring the recordingmedium 2 up to the interior of the stocker 7, there is a possibility ofgenerating the collapse of image due to the impingement among therecording medium 2 in the interior of the stocker 7. Therefore, if theink layer is preliminarily heated by the heater 6, the recording medium2 can be individually heated prior to the introduction thereof into thestocker 7, thus making it possible to prevent the generation of such acollapse of image.

As described above, when the inkjet ink according to the embodiments ofthe present invention is employed, the ink layer can be rapidlynon-fluidized by the application of irradiation of light and heatingafter finishing the delivery of ink onto printing matter. Namely, it ispossible to obtain a printed matter of high quality withoutnecessitating a large scale exposure system. Incidentally, thenon-fluidized film thus obtained should preferably be thermoplastic(i.e. it can be refluidized by the application of heat, so that there isno problem even if the retention time of this plasticity may berelatively short).

Moreover, in different from the conventional inkjet ink utilizingradical polymerization, it is not required, in the case of this inkjetink, to employ a carcinogenic radical-generating agent or a radicalpolymeric monomer which is highly stimulating to skin and badlysmelling. Therefore, the inkjet ink according to the embodiments of thepresent invention can be handled quite easily.

Further, in the case of the inkjet ink according to the embodiments ofthe present invention, typically, almost all of the solvents areconstituted by acid-polymerizable compounds. When the ratio of thepolymeric compounds in the inkjet ink to the entire volume of solventsis sufficiently high, there is little possibility of causing organicsolvent to volatilize on the occasion of printing. Therefore, it is nowpossible to obviate the problem of environmental contamination that mayoccur due to the evaporation of organic solvents, thus unnecessitatingthe provision of an exhaust installation or a solvent recoverymechanism.

Since the employment of organic solvent is no longer required and at thesame time, the ink layer can be rapidly non-fluidized, it is nowpossible to easily fix a printed image without substantially generatingbleeding on the printing surface of various different characteristics.Additionally, there is little possibility of the deterioration ofprinted surface which may otherwise occur due to the drying of the inklayer. Moreover, according to the inkjet ink of the embodiments of thepresent invention, since it is possible to incorporate pigment as acolor component at a high concentration, it is now possible to form aprinted pattern which is clear and excellent in weather resistance.

Next, the present invention will be further explained in detail withreference to specific examples.

Incidentally, the vinyl ether compounds to be set forth below areidentical in meaning with the compounds represented by the generalformula (1).

First of all, as acid-polymerizable compounds, vinyl ether compoundsrepresented by the following chemical formulas were prepared.

These vinyl ether compounds were employed according to the recipe shownin the following Tables 1 and 2 to obtain Solvents A1-A13, B1-B18, C1-C6and D1. TABLE 1 Vinyl ether compounds Epoxy compound Oxetane compoundsPropylene carbonate Ink Solvents (part by weight) (part by weight) (partby weight) (part by weight) No. A1 HQDV (100) 1 A2 4CHDV (100) 2 A31,3CH-DVE(100) 3 A4 IBVE (100) 4 A5 ISB-DVE(100) 5 A6 CHMVE(100) 6 A74CHDV (50) SR-NPG(50) 7 A8 TBP-VE(50) OXT221(50) 8 A9 HQDV (50)OXT221(50) 9 A10 4CHDV (50) OXT221(50) 10 A11 4CHDV (90) PC(10) 11 A124CHDV (50) Solubility parameter 20 or more (50) 12 A13 4CHDV (60)Viscosity 500 mPa · s or more(40) 13 A14 IBA(50) TPGDA(25) TMPETA(15) 14A15 DEGV(100) 15 A16 C3000(100) 33 B1 ONB-DVE(100) 34 B2 OCN-DVE(100) 35B3 M-CHDV(100) 36 B4 MEDVE(100) 37 B5 M-NBDV(100) 38 B6 NBDV(100) 39 B7ISB-DVE(80) 40 M-CHDV(20) B8 ISB-DVE(80) 41 MEDVE(20) B9 ONB-DVE(80) 42M-CHDV(20) B10 ONB-DVE(80) 43 MEDVE(20) B11 ISB-DVE(80) SR-NPG(20) 44B12 ISB-DVE(80) OXT221(20) 45 B13 ONB-DVE(80) SR-NPG(20) 46 B14ONB-DVE(80) OXT221(20) 47 B15 ISB-DVE(80) C3000(20) 48 B16 ISB-DVE(80)C2021(20) 49 B17 ONB-DVE(80) C3000(20) 50 B18 ONB-DVE(80) C2021(20) 51

TABLE 2 Vinyl ether compounds Epoxy compound Oxetane compounds Propylenecarbonate Ink Solvents (part by weight) (part by weight) (part byweight) (part by weight) No. C1 TBMP-IP(60) 52 ONB-DVE(40) C2 CHM-IP(40) 53 ONB-DVE(60) C3 DOM-IP(40) 54 ONB-DVE(60) C4 NBM-IP(40) 55ONB-DVE(60) C5 Formula Aro. 13(60) 56 ONB-DVE(40) C6 Formula Aro. 14(60)57 ONB-DVE(40) D1 DCPVE(60) 58 4CHDV(40)

The vinyl ethers shown in above Tables represent the followingcompounds.

-   -   HQDV: p-vinyloxy benzene    -   4CHDV: 1,4-cyclohexanediol divinylether    -   1,3CHDVE: 1,3-cyclohexanediol divinylether    -   IBVE: isoborneol vinylether    -   CHMVE: 1,4-cyclohexanedimethanol vinylether    -   TBP-VE: 1-tert-butyl-4-vinyloxy benzene    -   M-CHDV: 2-methoxy-1,4-cyclohexanediol divinylether    -   MEDVE: methoxy-vinyloxy-cyclohexanemethanol vinylether    -   M-NBDV: methoxynorbornane vinylether    -   ISB-DVE: isosorbite divinylether    -   ONB-DVE: hydroxymethyl-hydroxyoxanorbornanediol divinylether    -   OCN-DVE: oxabicyclononane divinylether    -   NBDV: norbornane divinylether    -   TBMP-IP: t-butylbenzylalcohol isopropenyl    -   CHM-IP: cyclohexanemethanol isopropenyl    -   DOM-IP: dioxoranemethanol isopropenyl    -   NBM-IP: norbornanemethanol isopropenyl    -   DCPVE: DCP dimethanol divinylether

In above Table 1, SR-NPG represents diglycidyl ether having a neopentylskeleton (Sakamoto Yakuhin Co., Ltd.), OXT-221 represents an oxetanecompound (Toa Gosei Co., Ltd.), C3000 represents limonene dioxide(Daicel chemical Industries, Ltd.), and DEGV represents diethyleneglycolmonovinyl ether (Maruzen Petrochemicals Co., Ltd.).

To each of the solvents A1 to A13, carbon black (5% by weight) as apigment, and UVACURE1591 (8% by weight) as a photo-acid generating agentwere added to obtain a mixture which was then subjected to dispersiontreatment a whole day and night by using a paint shaker. The mixturethat had been subjected to this dispersion treatment was filtered byusing a 5 μm PTFE filter to obtain inkjet inks (Nos. 1-13).

Incidentally, carbon black was kneaded together with acrylic resin-baseddispersing agent in advance to prepare a mill base, to which 200 ppm ofa nonionic surfactant (Sumitomo 3M Co., Ltd.) and a dispersing agentavailable in the market (Ajisper trade name; Ajinomoto Fine Techno Co.,Ltd.) were added to obtain inkjet inks (Nos. 1-13). Since these inkjetinks were all formulated such that not less than 40% by weight of theacid-polymerizable compounds was constituted by a vinyl ether compoundhaving a cyclic skeleton, these inks were all considered to belong tothe inkjet inks according to the embodiments of the present invention.Further, in the same manner as in the case of the inks of Nos. 1-13, theinks of Nos. 34-58 were prepared.

For the purpose of comparison, various photo-radical generating typeinkjet ink were prepared. First of all, 50 parts by weight of isobornylacrylate (IBA), 25 parts by weight of tripropylene glycol diacrylate(TPGDA) and 25 parts by weight of trimethylolpropane ethoxylatetriacrylate (TMPETA) were mixed together to prepare a solvent. To thissolvent, 5 parts by weight of Ilgacue (trade name) (NO.2959; NagaseSangyo Co., Ltd.), 5 parts by weight of carbon black pigment, a minuteamount of acrylic dispersing agent and a surfactant were added to obtaina mixture. This resultant mixture was then subjected to dispersiontreatment for 5 hours by using a homogenizer. This dispersed mixture wasthen filtered by using a 5 μm PTFE filter to obtain a photo-radicalgenerating type inkjet ink, thus forming No. 14 inkjet ink.

Additionally, a vinyl ether compound of non-cyclic skeleton (DEGV) andC3000 were respectively employed as a solvent and carbon black pigmentwas mixed as a color component with each of the solvents to prepare No.15 inkjet ink and No. 33 inkjet ink.

Since these inks of No. 14, No. 15 and No. 33 were formulated such thatthe vinyl ether compound having a cyclic skeleton was not includedtherein, they were considered as inks of comparative examples.

The properties of the inkjet inks thus obtained were evaluated by usingan inkjet recording apparatus shown in FIG. 1. As for the recordingmedium 2, an ordinary enameled paper was employed, and as for the lightsource 5, an ultra-high pressure mercury lamp having an output of 230 Wwas employed. Further, without employing a heater as a heating means 6,only the irradiation of ultraviolet ray was tried to cure an ink layer.Under these conditions, the inkjet recording was performed toinvestigate the pencil hardness, printing quality and solvent resistanceof the cured ink layer as well as the adhesion of ink layer to therecording medium.

Incidentally, the results of the pencil hardness shown in the followingtables were obtained by measuring the ink layer under the followingconditions.

-   -   Immediately after exposure: The ink layer immediately after        exposure;    -   Stocker: The ink layer which was left to stand for 3 minutes        inside the stocker at a temperature of 80° C. after the exposure        thereof;    -   Left standing at ordinary temperature: The ink layer which was        left to stand for 8 hours at the ordinary temperature (25° C.)        after the exposure thereof.

The quality of printing was evaluated by visually observing the printedimage, and the printed image which was free from missing therein wasdefined as “good”, and the printed image accompanying several portionsof missing was defined as “defective”.

The solvent resistance was evaluated as follows after performing anabrasiveness test using a sheet of cloth impregnated with an organicsolvent such as ethanol, acetone, etc.

-   -   A (very good): Nothing was changed;    -   B (good): Ink was adhered onto the cloth;    -   C (more or less defective): The color of ink layer became        thinner;    -   D (defective): Part of the ink layer was completely peeled away.

The adhesion was evaluated by performing a crosscut tape peeling test.Namely, when the area peeled of the ink layer is less than 10% based onthe entire area of the ink layer was defined as good, and when thispeeled area is not less than 10% based on the entire area of the inklayer was defined as defective.

First of all, the influence of the content of the vinyl compounds in theacid-polymerizable compounds on the properties of ink was investigated.Various inks were prepared according to the same recipe as that of No. 2inkjet ink shown in above Table 1 except that, as part of theacid-polymerizable compounds, an epoxy compound (C3000) was incorporatedat a predetermined ratio into the ink. The inks thus obtained wereapplied to the recording apparatus shown in FIG. 1 to investigate thepencil hardness and solvent resistance of the cured ink layer.

Likewise, various inks were prepared according to the same recipe asthat of No. 45 inkjet ink or of No. 46 inkjet ink shown in above Table 1except that, as part of the acid-polymerizable compounds, an oxetanecompound (OXT221) and an epoxy compound (SR-NPG) were incorporated at apredetermined ratio into the ink.

The results obtained are summarized in the following Tables 3 to 5together with the content of the vinyl ether (VE) compound. TABLE 3Content of VE (part by weight) 10 20 30 35 40 50 60 100 Pencil hardnessF F F F H H 2H 2H Exposure 300 300 300 300 250 250 250 250 (mJ/cm²)Solvent More or More or More or Good Very Very Very Very resistance lessless less good good good good defective defective defective

TABLE 4 Content of VE (part by weight) 0 5 10 20 30 40 50 60 Pencil HBHB F F F H H 2H hardness Exposure 400 270 260 260 260 250 250 250(mJ/cm²) Solvent More or Good Good Good Good Good Good Good resistanceless defective

TABLE 5 Content of VE (part by weight) 0 5 10 20 30 40 50 60 Pencil 2BHB F F F H H H hardness Exposure 500 280 270 270 270 260 260 260(mJ/cm²) Solvent Defective Good Good Good Good Good Good Good resistance

As shown in above Tables 4 and 5, it was confirmed that even if a littleamount of a vinyl ether compound having a cyclic skeleton wasincorporated into a solution of ink, it was possible to prominentlyenhance the photosensitivity of ink, and it was also effective inenhancing the solvent resistance of the ink. Further, as shown in aboveTables 3, 4 and 5, it was also confirmed that when a vinyl ethercompound having a cyclic skeleton was incorporated into a solution ofink at a ratio of 40% by weight of the acid-polymerizable compounds, itwas possible to prominently enhance the photosensitivity of ink, and thesolvent resistance of the cured ink layer thus formed was alsoexcellent. Even if the cured ink layer was subjected to rubbing using asheet of cloth impregnated with an organic solvent such as ethanol,acetone, any change was not recognized in the cured ink layer, thusconfirming that the cured ink layer was excellent in solvent resistance.

Furthermore, by using the inkjet inks, the recording of images on arecording medium such as an OHP sheet was performed to investigate thecuring of ink. As a result, it was confirmed that, when inkjet inksaccording to the embodiments of the present invention were employed, itwas possible to form a cured ink film excellent in adhesive strength.

The following Table 6 illustrates the results wherein the properties ofthe inkjet inks of No. 2, No. 14 and No. 33 were compared with eachother. TABLE 6 Pencil hardness Ink Exposure After Standing at PrintingSolvent No. (mJ/cm²) exposure Stocker normal temp. quality resistanceAdhesion 2 250 H 2H 2H Good Good Good 14 1750 Un-cured Un-cured Un-curedDefective Defective Defective 33 250 HB F F Good More or less Gooddefective

As shown in above Table 6, No. 2 inkjet ink according to one embodimentof the present invention was capable of exhibiting a pencil hardness ofH or more immediately after the exposure thereof even it was irradiatedwith light at an exposure quantity of as very low as 250 mJ/cm².Moreover, the cured ink layer thus obtained was also excellent in all ofprinting quality, solvent resistance and adhesion to recording medium.

Whereas, in the case of No. 14 ink which was photo-radical generatingtype, even if it was irradiated with light by increasing the exposuredosage up to 1750 mJ/cm², and at the same time, the resultant ink layerwas heated in the stocker, it was impossible to cure the ink layer.Further, in order to enable the ink layer to cure at an exposure dosageof 1000 mJ/cm², the concentration of the pigment in the inkjet ink wasrequired to be decreased to 3% by weight of the ink. The pencil hardnessof the ink layer on this occasion was at most HB and the printed imageobtained was weak in color and strong in gloss of resin, thus making itunsuitable for practical use.

In the case of No. 33 ink containing only an epoxy resin as anacid-polymerizable compound, even if it was possible to achieve a pencilhardness of HB at an exposure dosage of 250 mJ/cm², it was impossible toachieve a pencil hardness of H as in the case of No. 2 inkjet ink.Further, the printed image thus obtained was slightly glossy even if theimage itself was good.

It was confirmed through the comparison among these inkjet inks that No.2 ink according to one embodiment of the present invention was capableof achieving a high hardness even if only the irradiation of ultravioletray was employed for the curing, and that No. 2 ink was very excellentin quick-drying property.

The following Table 7 illustrates the results wherein the properties ofthe inkjet inks of No. 2 and No. 15 were compared with each other. TABLE7 Pencil hardness Ink Exposure After Standing at Printing Solvent No.(mJ/cm²) exposure Stocker normal temp. quality resistance Adhesion 2 250H 2H 2H Good Good Good 15 400 B HB HB Good Defective Defective

In No. 2 inkjet ink, the acid-polymerizable compound was constituted bya vinyl ether compound having an alicyclic skeleton (4CHDV), and in No.15 inkjet ink on the other hand, the acid-polymerizable compound wasconstituted by a vinyl ether compound having a chain skeleton (DEGV).

As shown in above Table 7, No. 2 inkjet ink was capable of being curedat an exposure dosage which was lower than that employed in No. 15inkjet ink by about 150 mJ/cm² and moreover, the cured ink layerobtained therefrom was also excellent in solvent resistance and adhesionto recording medium. This preferable trend was also confirmed in thecases wherein the inks were prepared using vinyl compounds having otheralicyclic skeleton as an acid-polymerizable compound. Therefore, it hasbeen found out that when these vinyl compounds were employed for thepreparation of an inkjet ink, it is possible to obtain an ink veryexcellent in curability.

Based on these results described above, it has been determined that atleast 40% by weight of acid-polymerizable compounds should beconstituted by a vinyl compound having a cyclic skeleton in theembodiments of the present invention. In the followings, the resultsinvestigated of the influence of the cyclic skeleton are illustrated.

The following Table 8 illustrates the results wherein the properties ofthe inkjet inks of No. 1 and No. 2 were compared with each other. TABLE8 Pencil hardness Ink Exposure After Standing at Printing Solvent No.(mJ/cm²) exposure Stocker normal temp. quality resistance Adhesion 1 250H 2H 2H Good Very good Good 2 250 H 2H 2H Good Good Good

In No. 1 inkjet ink, the vinyl ether compound employed therein wasprovided with aromatic ring skeleton (benzene ring) as a cyclicskeleton, and in No. 2 inkjet ink on the other hand, the vinyl ethercompound employed therein was provided with an alicyclic skeleton(cyclohexane ring) as a cyclic skeleton. As compared with No. 2 inkjetink, No. 1 inkjet ink was capable of being cured at almost the sameexposure dosage as that of No. 2 inkjet ink, and the hardness andadhesion of the cured ink layer to be achieved by the employment of No.1 inkjet ink were almost the same as those of No. 2 inkjet ink. It willbe also found from the results of above Table 8 that No. 1 inkjet inkwas very excellent in solvent resistance. The reason for this was founddue to the characteristics of ink using a vinyl ether compound having anaromatic ring skeleton, which is highly resistive to various chemicalsincluding oily hydrocarbon solvents such as ethanol and acetone, andester-based solvents.

The following Table 9 illustrates the results wherein the properties ofthe inkjet inks of No. 4 and No. 5 were compared with each other. TABLE9 Pencil hardness Solvent Ink Exposure After Standing at resis- Ad- No.(mJ/cm²) exposure Stocker normal temp. tance hesion 4 250 F H H GoodGood 5 250 F H H Good Good

In No. 4 inkjet ink, the vinyl ether compound (IBVE) employed thereinwas provided with a cyclic skeleton containing no oxygen atom as aring-constituting atom, and in No. 5 inkjet ink on the other hand, thevinyl ether compound (ISB-DVE) employed therein was provided with acyclic skeleton containing oxygen atom as a ring-constituting atom. Dueto this difference in ring structure, ISB-DVE was capable of exhibitingan extremely high polarity as compared with IBVE.

It will be seen from the results shown in FIG. 9 that there was littledifference in curing property between No. 4 inkjet ink and No. 5 inkjetink. However, when an image was formed on a recording medium exhibitinga relatively high polarity such as a PET film, a phenomenon wasrecognized that there was a substantial difference in wettabilitybetween them. Specifically, in the case of No. 4 inkjet ink, there wasrecognized a phenomenon of bleeding where droplets of ink exhibitingpoor wettability to a recording medium were repelled, thus deterioratingthe quality of printed image as compared with No. 5 inkjet ink. Thisdifference can be attributed to a difference in polarity between thesevinyl ether compounds.

Next, the mixing ratio between IBVE and ISB-DVE was variously altered inthe preparation of inkjet inks, which were then employed for formingimages on a PET film. The quality of images thus obtained was comparedwith other, the results being illustrated in the following Table 10.TABLE 10 IBVE:ISB-DVE 10:1 8:2 6:4 Image quality Yes Slight None(bleeding) bleeding

As shown in above Table 10, it was possible, due to the incorporation ofISB-DVE, to improve the wettability of the ink and to enhance thequality of printed image. As long as a vinyl ether compound is providedwith a cyclic skeleton containing hydrogen atom as a ring-constitutingatom, the vinyl ether is enabled to have the same degree of highpolarity as that of ISB-DVE, so that the vinyl ether is enabled to havethe similar properties as those of ISB-DVE. Accordingly, it wasconfirmed that it was possible, through the employment of such a vinylether compound, to control the wettability thereof to recording mediumwithout deteriorating the curing property of ink and hence theemployment of such a vinyl ether compound is quite useful as a solventfor inkjet ink.

The following Table 11 illustrates the results wherein the curingcharacteristics of the inkjet inks of No. 2 and No. 11 were comparedwith each other. No. 11 ink was formed of the same composition as thatof No. 2 ink except that propylene carbonate was incorporated therein ata ratio of 10% by weight of the acid-polymerizable compounds. TABLE 11Pencil hardness Ink Exposure After Standing at Printing Solvent No.(mJ/cm²) exposure Stocker normal temp. quality resistance Adhesion 2 250H 2H 2H Good Good Good 11 500 HB F F Good Good Good

As shown in above Table 11, although No. 11 ink was capable ofexhibiting the same properties as those of No. 2 ink with respect toprinting quality, solvent resistance and adhesion, the hardness of thecured ink layer thus obtained was lower than that of No. 2 ink. Further,since the dosage of exposure required in No. 11 ink was larger than thatin No. 2 ink so that the sensitivity thereof to light was naturally low.Accordingly, it is inconceivable in view of the results of above Table11 that the incorporation of propylene carbonate would bring about anyespecially excellent effect.

Next, various inkjet inks were prepared so as to have the samecompositions as those of the aforementioned No. 2 ink and No. 11 inkexcept that the mixing ratio of the photo-acid generating agent wasaltered. Then, these inkjet inks were investigated with respect to thecuring property of ink, the presence or absence of precipitated solidmatters, and delivery performance. The results thus obtained aresummarized in the following Table 12. Incidentally, the precipitation ofsolid matters was visually observed. The delivery performance wasevaluated by visually confirming the printed image wherein the printedimage which was free from missing of image was defined as “good”, andthe printed image accompanying several missing portions was defined as“defective”. TABLE 12 Content of photo-acid Precipitation Ink generatingExposure Pencil hardness of solid Delivering No. agent (mJ/cm²) afterexposure matters performance Shelf life 2  8% 250 H None Good Good 10%250 H Yes Defective Defective 12% 250 H Yes Defective Defective 11  8%500 HB None Good Good 10% 400 F None Good Good 12% 300 H None Good Good

As shown in above Table 12, in the case of No. 2 ink containing nopropylene carbonate, when the content of the photo-acid generating agentbecame 10% or more, the photo-acid generating agent precipitated assolid matters in the ink even though the curing property was notchanged. The inkjet ink having the photo-acid generating agentprecipitated therein was very poor in delivery performance.

On the other hand, in the case of No. 11 ink, even if the mixing ratioof the photo-acid generating agent was increased, the precipitation ofsolid matters was not recognized, and still more, the curing propertywas improved. As a result, it was possible to maintain an excellentstorage property.

Generally speaking, even though it is possible to improve the curingperformance of ink by increasing the content of the photo-acidgenerating agent, it has been impossible to prevent the deterioration ofthe storage property of ink. However, it has been confirmed possible toretain stable properties of ink without deteriorating the storageproperty thereof by the addition of propylene carbonate even if the inkis preserved for a long period of time.

The inkjet ink according to the embodiments of the present invention isrequired to comprise a vinyl ether compound which is one ofacid-polymerizable compounds. By incorporating a predetermined compoundas part of the acid-polymerizable compounds into an inkjet ink, it wouldbecome possible to provide the inkjet ink with desired properties.

The following Table 13 illustrates the results wherein the properties ofthe inkjet inks of No. 2 and No. 10 were compared with each other. TABLE13 Pencil hardness Ink Exposure After Standing at Printing Solvent No.(mJ/cm²) exposure Stocker normal temp. quality resistance Adhesion 2 250H 2H 2H Good Good Good 10 250 2H 3H 3H Good Very good Good

No. 10 inkjet ink was formed of the same composition as that of No. 2inkjet ink except that 50% by weight of the acid-polymerizable compoundswas constituted by an oxetane compound (OXT221) . As shown in aboveTable 13, this oxetane compound was employed together with a vinyl ethercompound to enhance the curing hardness. Thus, it was confirmed that theincorporation of an oxetane compound was effective for obtaining a curedink film more excellent in toughness. Furthermore, since the oxetanecompound was also provided with an effect to reduce the volatilizationrate of ink, it was also possible to improve the re-delivery performance(delivery performance when the action of delivery is repeated withoutperforming the delivery for a predetermined period of time).

The following Table 14 illustrates the results wherein the properties ofthe inkjet inks of No. 2 and No. 7 were compared with each other. Theplasticity of the ink layer was evaluated by the cracking test of thecured ink layer. Namely, the cured ink layer was quenched from 50° C.down to −50° C. to see if there was any cracking of the ink layer,wherein the ink layer where a very small degree of cracking was visuallyrecognized was defined as “good”, and the ink layer where cracking wasvisually not recognized at all was defined as “very good”. TABLE 14Pencil hardness Ink Exposure After Standing at Solvent No. (mJ/cm²)exposure Stocker normal temp. Printing quality resistance Adhesion 2 250H 2H 2H Good Good Good 7 300 F 2H 2H More or less good Very good Verygood

No. 7 inkjet ink was formed of the same composition as that of No. 2inkjet ink except that 50% by weight of the acid-polymerizable compoundswas constituted by a diglycidyl ether compound (SR-NPG). As shown inabove Table 14, it was possible, through the employment of the oxetanecompound together with a vinyl ether compound, to enhance the plasticityand storage property of the cured ink layer without deteriorating thecuring hardness at all.

Next, the influence of the structure of the vinyl ether compounds to beincorporated in the ink as an acid-polymerizable compound on theproperties of ink was investigated. The following Tables 15 and 16illustrate the results wherein the properties of the inkjet inks of Nos.1-6 and Nos. 34-58 were compared with each other. TABLE 15 Pencilhardness Ink Exposure After Standing at Printing Solvent No. (mJ/cm²)exposure Stocker normal temp. quality resistance Adhesion 1 250 H 2H 2HGood Good Good 2 250 H 2H 2H Good Good Good 3 250 H 2H 2H Good Good Good4 270 F H H Good More or Good less good 5 250 H 2H 2H Good Good Good 6400 F H H Good More or Good less good 34 220 2H 3H 3H Good Good Good 35260 F F H Good Good Good 36 230 F H H Good Good Good 37 250 F H H GoodGood Good 38 250 F H H Good Good Good 39 250 F H H Good Good Good 40 250H 2H 2H Good Good Good 41 250 H 2H 2H Good Good Good 42 250 H 2H 2H GoodGood Good 43 250 H 2H 2H Good Good Good 44 280 F F H Good Good Good 45250 H 2H 2H Good Good Good 46 260 F H H Good Good Good 47 240 H 2H 2HGood Good Good 48 250 F H H Good Good Very good 49 250 F H H Good GoodVery good 50 250 H H H Good Good Very good 51 250 H H H Good Good Verygood

TABLE 16 Pencil hardness Ink Exposure After Standing at Printing SolventNo. (mJ/cm²) exposure Stocker normal temp. quality resistance Adhesion52 300 F H H Good More or More or less good less good 53 270 F H H GoodMore or More or less good less good 54 280 F H H Good More or More orless good less good 55 280 F H H Good More or More or less good lessgood 56 250 H 2H 2H Good Good Good 57 300 F H 2H Good Good Good 58 250 HH H Good Good Good

Among the vinyl ether compounds incorporated into these inkjet inks,IBVE(No. 4) and CHMVE(No. 6) were formed of a monovalent vinyl ethercompound, and the rest were formed of a bivalent vinyl ether compound.As shown in above Table 15, the inks (Nos. 1-3, No. 5 and Nos. 34-39)containing a bivalent vinyl ether compound were curable with an exposuredosage of as low as several hundreds to several tens mJ/cm² and thepencil hardness of the cured layer was excellent in general. It isassumed that these preferable performance can be obtainable even ifvinyl ether compounds having a valency of 2 or more were employed.Accordingly, even if vinyl ether compounds having a valency of 3 or morewere employed, it would be possible to enhance the sensitivity of ink tolight. In particular, No. 34 ink was very excellent in photosensitivecuring property among the inks which were prepared using a singlesolvent, thus expressing the characteristic performance of ONB-DVEsolvent. This excellent photosensitive curing property of No. 34 inkwill be clearly recognized even in the comparison thereof with theaforementioned No. 10 ink containing an oxetane compound. As far as theinks which are prepared using a single solvent are concerned, theexistence of the ink having such an excellent photosensitivity would bequite rare. Accordingly, it is now possible to prepare an ink havingvarious properties useful of course for printing and also exhibitingadvantages in terms of manufacturing cost.

The inks of Nos. 40-43 represent examples of inks which were preparedusing solvents into which vinyl ether compounds were incorporated.ISB-DVE and ONB-DVE were formed of a structure comprising an alicyclicskeleton having oxygen atom included therein, and M-CHDV and MEDVE wereformed of a structure comprising an alicyclic skeleton havingoxygen-containing substituent group included therein. These solvents arecharacterized in that they provide an excellent hardness of cured filmthereof, resulting also in excellent photosensitivity of ink.Additionally, since these solvents were excellent in terms ofvolatility, it was possible to realize very excellent delivery actionand delivery stability.

The inks of Nos. 44-51 represent examples of inks which were prepared bymixing ISB-DVE and ONB-DVE as vinyl ether compounds with a solventcomprising an epoxy compound and an oxetane compound. It will berecognized that in addition to high photo-sensitivity which all of theseinks have, No. 44 ink and No. 46 ink were excellent in flexibility ofthe cured film thereof. It will be recognized that No. 45 ink and No. 47ink were excellent in curing hardness. Further, it will be recognizedthat inks of Nos. 48-51 were more excellent in adhesion to recordingmedium.

The inks of Nos. 52-55 represent examples of inks which were prepared bymixing ONB-DVE with isopropenyl ether compound. It will be recognizedthat in addition to high photo-sensitivity which all of these inks have,isopropenyl ether compound was relatively low in volatility and henceexcellent in stability in delivery of ink. Further, these inks werefound excellent in dissolving an acid-generating agent.

The inks of Nos. 56 and 57 represent examples of inks wherein ONB-DVEand a vinyl ether compound having an aromatic ring were employed. Theseinks were excellent in photosensitive curability, and in dissolving anacid-generating agent. Due to the introduction of an aromatic ring,these inks were enhanced in curability. Further, due to the introductionof methoxy group, the polarity thereof was increased. As a result, theseinks were excellent in solubility and volatility, and at the same time,these inks were capable of suppressing the evaporation of the solvent,thus enabling these inks to exhibit more stable deliverability. It waspossible, with the employment of these inks, to improve the wettabilityof ink to a printing medium and hence to obtain a printed matterenhanced in quality of printed images.

Further, CHMVE is a vinyl ether compound having a cyclohexyl skeleton.In the case of an ink comprising a vinyl ether compound (DCPVE) havingalicyclic composite cyclic skeleton and linked via a methylene akeletonto vinyl ether group, it is more excellent in photosensitivity ascompared with the ink where CHMVE is employed. It was possible toconfirm this from the results of the ink of No. 58. As compared with avinyl ether compound having an alicyclic structure of monocyclicskeleton, a vinyl ether compound having a composite cyclic skeleton wasmore effective in providing an ink of excellent properties. This trendwas admitted also in the cases where a vinyl ether group is not directlylinked to an alicyclic skeleton.

Incidentally, in the case of vinyl ether compounds (HQDV, 4CHDV,1,3CH-DVE and ISB-DVE) which were employed in the inks of Nos. 1-3 andNo. 5, the vinyl ether group is directly linked to the cyclic skeleton.The vinyl ether compound (CHMVE) employed in No. 6 ink was provided witha methylene skeleton. It will be recognized from the comparison of themthat the ink containing a vinyl ether compound where vinyl ether groupis directly linked to a cyclic skeleton is more excellent inphotosensitivity as compared with the ink containing a compound having amethylene skeleton. In particular, it is possible in the case of theformer ink to enhance the properties such as solvent resistance andadhesion. It was also possible to recognize a trend that as not only themethylene skeleton but also the structure of the linkage portion betweenthe vinyl ether group and the cyclic skeleton became larger, thephotosensitivity of ink decreased. It was also confirmed that a vinylether compound having vinyl ether group directly linked to a cyclicskeleton was capable of exhibiting very excellent properties.

ISB-DVE and ONB-DVE employed in the inks of No. 5 and No. 34 arecompounds having a distorted cyclic ether structure such as oxetane ringor hydrofuran ring. These compounds are considered excellent inreactivity due to their specific structure. In particular, when thesecyclic structures are provided with a bridged structure, it is possibleto provide the ink with most preferable properties.

Next, the influence of the content of photo-acid generating agent wasinvestigated. An inkjet ink was prepared by following the sameprocedures as those of No. 2 ink except that the mixing ratio of thephoto-acid generating agent was altered as shown in the following Table17. By using this ink, the performance test thereof was performed usingthe inkjet recording apparatus 1 shown in FIG. 1. The quantity ofexposure, viscosity and the state of delivery of ink on this occasionare summarized in Table 17 together with the mixing ratio of thephoto-acid generating agent. The state of delivery of ink was determinedfrom the fluctuation in the state of delivery of ink after the ink wassubjected to an accelerated test which was performed for one week undera heated condition of 60° C. TABLE 17 Content of photo-acid generatingagent (wt %) 1 2 4 6 8 10 Exposure (mJ/cm²) 800 400 300 250 250 250Viscosity (mPa · s) 10 11 13 15 22 37 Delivering condition Good GoodGood Good Barely Impossible possible Corrosion of Ni None None TraceLittle Yes Grate

As shown in Table 17, when the mixing ratio of the photo-acid generatingagent was 1% by weight, it required an exposure dosage of 800 mJ/cm²,thus indicating a low sensitivity of ink. On the other hand, when themixing ratio of the photo-acid generating agent was increased over 10%by weight, the viscosity of ink gradually increased, resulting incomplete failure of delivery. However, when the mixing ratio of thephoto-acid generating agent was 2% by weight, it was found possible toretain satisfactory properties of ink such as sufficient curinghardness, solvent resistance, etc., thus confirming that the inkcontaining the vinyl ether compound was very excellent inphotosensitivity.

Above Table 17 also shows the corrosive properties of Ni metal as Nimetal was immersed in each of the inks. Ni metal is generally used inpiping or electrodes. Therefore, it is desired that the corrosion of Nimetal by the effect of ink should be avoided as much as possible. Theresults of above Table 17 clearly shows that when the mixing ratio ofthe photo-acid generating agent was increased up to 8% by weight ormore, the corrosion of Ni metal would become prominent.

In the following Table 18, the relationship between the viscosity ofinkjet ink and the delivery performance thereof is summarized. In thiscase, various inks were prepared by adjusting the composition of No. 8ink so as to set the viscosity thereof to a predetermined value. Then,the properties of each of the inks were investigated by performing thedelivery operation of these inks, the results being illustrated in thefollowing Table 18. Incidentally, the control of temperature was notperformed on the occasion of delivering the inks. TABLE 18 Viscosity ofink (mPa · s) 10 11 12 13 14 15 Delivering Good Good Good More orDefective Impossible performance less good without temp. control

Generally, it is required to control the temperature on the occasion ofperforming the delivering of ink through a head. The results of Table 18show that as long as the viscosity of ink can be controlled to 12 mPa.sor less, the ink can be delivered or discharged without necessitatingsuch a temperature control, thereby facilitating the designing of head.

Then, the influence of the solubility parameter (SP value) of the epoxycompound was investigated, the results being summarized in the followingTable 19. In this case, a plurality of inkjet inks were prepared byfollowing the same procedures as those of No. 12 ink except that anepoxy compound having a predetermined solubility parameter wasincorporated in the inks. These inks were investigated with respect tothe solubility of the vinyl ether compound, the solubility of thephoto-acid generating agent, and the performance of delivery of ink, theresults being illustrated in the following Table 19. Incidentally, thesolubility was evaluated by visually observing a solution to be obtainedas predetermined acid-polymerizable compounds and photo-acid generatingagents were mixed therein, wherein a solution which is completely freefrom turbidity and precipitates was defined as “good”, and a solutionaccompanying turbidity or containing a little amount of precipitates wasdefined as “slightly defective”. A solution exhibiting an intermediatestate between “good” and “slightly defective” was defined as “more orless good”. TABLE 19 SP value (Mpa^(1/2)) 17 18 19 20 21 22 Solubilityof Good Good Good Good Good Good vinyl ether Solubility of Poor More orless Good Good Good Good photo-acid good generating agent DeliveringImpossible More or less Good Good Good Good performance good

As shown in Table 19, the inks containing an epoxy compound exhibiting asolubility parameter of 19 MPa^(1/2) or more were found capable ofeasily dissolving the vinyl ether compound and the photo-acid generatingagent. Whereas, in the case of the inks exhibiting a solubilityparameter of less than 19 MPa^(1/2), the photo-acid generating agentprecipitated as solid matters and to remain in the inks as impurities.Since these impurities would badly affect the quality of ink, such asdeterioration of ink-delivering performance, deterioration ofphotosensitivity of ink, and deterioration of filtering conditions inthe process of manufacturing the ink, it is desirable to minimize theseimpurities as much as possible.

Further, the influence of viscosity of the epoxy compound on the ink wasinvestigated, the results being summarized in the following Table 20. Inthis case, a plurality of inkjet inks were prepared by following thesame procedures as those of No. 13 ink except that an epoxy compoundhaving a predetermined viscosity was incorporated in the inks. Theseinks were investigated with respect to the viscosity and the deliveryperformance thereof, the results being illustrated in the followingTable 20. Incidentally, the following Table 21 shows the results of theinks which were prepared by incorporating a low viscosity epoxy compoundinto the inks in place of the vinyl ether compound. TABLE 20 Viscosityof epoxy compound (mPa · s) 100 500 1000 10000 Viscosity of ink 10 15 3545 (mPa · s) Delivering Good Good Good Good performance Glossiness PoorGood Good Good

TABLE 21 Viscosity of epoxy compound (mPa · s) 100 500 1000 10000Viscosity of ink 20 55 70 120 with epoxy compound (mPa · s) DeliveringGood Poor Impossible Impossible performance

As shown in Tables 20 and 21, it was possible, due to the employment ofthe vinyl ether compound, to obtain inks which were not only excellentin compatibility to an acid-polymerizable compound of high viscosity butalso low in viscosity. Moreover, the delivery performance was alsoexcellent in all of the inks thus obtained. Since it was possible toincorporate an acid-polymerizable compound of high viscosity into theseinks, the cured ink layer was enabled to have a glossy surface, and atthe same time, the ink layer was enhanced in flexibility. Whereas, inthe case of the inks into which an epoxy resin of low viscosity wasincorporated, it was difficult to sufficiently lower the viscosity ofink to such an extent that makes the ink meet suitable deliveryconditions, and the photosensitivity of the inks was also poor.

Solvents A2, A7 and A10 were respectively mixed with other componentsaccording to the recipe shown in the following Table 22 to preparevarious inkjet inks. TABLE 22 Photo-acid Viscosity- Solvents No.Pigments generating stabilizing (wt %) (wt %) agent(wt %) agent(tw %)Ink No. A2(78.7) BL(5%) PAG5(8) BS1(8.3) 16 A2(78.7) BL(5%) PAG5(8)BS2(8.3) 17 A2(78.7) BL(5%) PAG5(8) BS3(8.3) 18 A2(78.7) BL(5%) PAG5(8)BS4(8.3) 19 A2(78.7) BL(5%) PAG5(8) BS5(8.3) 20 A2(78.7) BL(5%) PAG5(8)BS6(8.3) 21 A2(78.7) BL(5%) PAG5(8) BS7(8.3) 22 A7(78.7) BL(5%) PAG5(8)BS3(8.3) 23 A10(78.7) BL(5%) PAG5(8) BS3(8.3) 24 A2(78.7) Y(5%) PAG5(8)BS5(8.3) 25 A2(78.7) M(5%) PAG5(8) BS5(8.3) 26 A2(78.7) C(5%) PAG5(8)BS5(8.3) 27 A7(78.7) BL(5%) PAG1(8) BS7(8.3) 28 A7(78.7) BL(5%) PAG2(8)BS7(8.3) 29 A7(78.7) BL(5%) PAG3(8) BS7(8.3) 30 A7(78.7) BL(5%) PAG4(8)BS7(8.3) 31 A2(92) PAG5(8) 32

Carbon black, C.I. Pigment Yellow 180, C.I. Pigment Blue 15 and C.I.Pigment Red 123 were employed for the preparation of black pigment (BK),yellow pigment (Y), cyan pigment (C) and magenta pigment (M),respectively.

Specific examples of the compounds employed as a photo-acid generatingagent are shown below.

Incidentally, PAG5 is a 50% propylene carbonate solution of a mixture ofPAG1 and PAG2.

Further, specific examples of the compounds (BS1-BS7) incorporated intothe inks as a viscosity-stabilizing agent are shown by the chemicalformulas thereof.

These compounds were added to the inks by mol % based on net photo-acidgenerating agent (for example, in the case of combination of BS andPAG5, it would be 8.3 mol % corresponding to about 1 wt %).

In the preparation of the inks, each pigment was kneaded together withacrylic resin-based dispersing agent in advance to prepare a mill base,to which 200 ppm of a nonionic surfactant (Sumitomo 3M Co., Ltd.) and adispersing agent available in the market (Ajisper trade name; AjinomotoFine Techno Co., Ltd.) were added to obtain a mixture. Further,according to the recipe shown in above Table 20, a pigment, a photo-acidgenerating agent and a viscosity-stabilizing agent were added to themixture and then subjected to dispersion treatment a whole day and nightby using a paint shaker. The resultant mixture that had been subjectedto this dispersion treatment was filtered by using a 5 μm PTFE filter toobtain inkjet inks (Nos. 16-31).

Meanwhile, the same procedures as described above were repeated exceptthat the pigment and the viscosity-stabilizing agent were notincorporated therein to obtain No. 32 ink. Likewise, the same proceduresas described above were repeated except that the viscosity-stabilizingagent was not incorporated therein to obtain No. 2 ink.

The inks prepared in this manner were left to stand for 6 months at theordinary temperature to measure the viscosity thereof, and based on theratio of increase of viscosity, the shelf life of the inks wasevaluated, the results being summarized in the following Table 23. Inthe determination of the ratio of increase of viscosity, the inks wereleft to stand for 6 months at the ordinary temperature, and theviscosity of the inks before and after this standing was measured todetermine the ratio of increase of viscosity. The ratio of increase ofviscosity was evaluated according to the following criterion. TABLE 23Ink Shelf No. life 16 A 17 A 18 A 19 A 20 A 21 A 22 A 23 A 24 A 25 A 26A 27 A 28 A 29 A 30 A 31 A 32 B 2 CA: 10% or lessB: 10 to 20%C: 20% or more

As shown in Table 23, in the case of the inks Nos. 32 and 2, the ratioof increase of viscosity was higher than 10%. It will be seen from theseresults that if a viscosity-stabilizing agent is not included in the ink(inks Nos. 32 and 2), the ratio of increase of viscosity would becomehigher.

Next, by applying each of the inks that had been left to stand for 6months under the aforementioned conditions to the inkjet recordingapparatus shown in FIG. 1, the quantity of exposure and the hardness ofthe cured ink layer were investigated. As for the recording medium 2,the ordinary enameled paper was employed. As for the light source 5, anultra-high pressure mercury lamp having an output of 230 W was employed.With respect to the properties of the inks, the pencil hardness of theink layer (the hardness of the ink layer that had been kept in a stockerfor 3 minutes at a temperature of 80° C. after finishing the exposure)and the quality of printing were measured. In the same manner, theproperties of the inks of the initial state thereof before the storagewere investigated, the results being illustrated in the following Table24 together with the quantity of exposure. TABLE 24 Initial time After 6month Ink Exposure Pencil Exposure Pencil Printing No. (mJ/cm²) hardness(mJ/cm²) hardness quality 16 300 H 300 H Good 17 300 H 300 H Good 18 300H 300 H Good 19 300 H 300 H Good 20 300 H 300 H Good 21 300 H 300 H Good22 300 H 300 H Good 23 300 H 300 H Good 24 300 H 300 H Good 25 300 H 300H Good 26 300 H 300 H Good 27 300 H 300 H Good 28 300 H 300 H Good 29300 H 300 H Good 30 300 H 300 H Good 31 300 H 300 H Good 32 250 2H — —Impossible

As shown in Table 24, in the case of the ink No. 2, the viscositythereof greatly increased after it was left to stand for 6 months, thusmaking it impossible to perform the delivery thereof. Whereas, in thecase of the inks Nos. 16-31 each containing a basic compound or abasicity-generating compound as a viscosity-stabilizing agent, they werecapable of being delivered in an excellent manner even after thestanding thereof for 6 months, thus indicating remarkably enhancedstabilization of viscosity.

When investigations were performed on the corrosive property of theseinks to Ni metal to be employed for the manufacture the piping or headmember of recording apparatus, the corrosion of these members was notrecognized at all. On the other hand, in the case of the inks Nos. 32and 2 where the viscosity-stabilizing agent was not included therein,some degree of corrosion was recognized, thus confirming that theviscosity-stabilizing agent was capable of functioning as ananti-corrosion agent.

Next, a plurality of inkjet inks having the same composition as that ofNo. 23 ink except that the mixing ratio (mol % based on the photo-acidgenerating agent) of the basic compound BS3 (i.e. aviscosity-stabilizing agent) was varied were prepared. These inks werethen measured with respect to the pencil hardness and the ratio ofincrease of viscosity after 6 months, the results obtained are shown inthe following Table 25 together with the ratio of the compound BS3.TABLE 25 Ratio of BS3 (molar ratio) 1 2 7 15 30 50 70 75 100 Pencilhardness H H F F B <2B — — — Shelf life C C B B A A A A A

As shown in Table 25, when the viscosity-stabilizing agent wasincorporated at a ratio of 1 mol % based on the quantity of thephoto-acid generating agent, the effect of the viscosity-stabilizingagent to stabilize the viscosity was not recognized in any substantialdegree. On the other hand, the viscosity-stabilizing agent wasincorporated at an excessive ratio of over 30 mol %, the curing propertyof ink deteriorated. It will be seen from these results that the mixingratio of the viscosity-stabilizing agent should preferably be confinedwithin the range of 1 to 30 mol % based on the quantity of thephoto-acid generating agent.

Furthermore, the influence of pKb of the basic compounds wasinvestigated. In this case, a plurality of inkjet inks were prepared inthe same manner as No. 16 ink except that the pKb of the basic compoundto be employed was altered as shown in the following Table 26. Theseinks were investigated with respect to the shelf life andphotosensitivity, the results being illustrated in Table 26. TABLE 26PKb 2 3 4 5 6 7 8 9 Shelf life A A A A A A B C Photosensitivity B A A AA A A A

The inks were individually left to stand for 6 months at the ordinarytemperature, and the viscosity of the inks before and after thisstanding was measured to determine the ratio of increase of viscosity,from which the ratio of increase of viscosity was evaluated according tothe following criterion.

-   -   A: 10% or less    -   B: 10 to 20%    -   C: 20% or more

The photosensitivity was investigated by measuring the quantity ofexposure required for enabling the cured ink layer to exhibit a pencilhardness of H and then the photosensitivity was evaluated according tothe following criterion.

-   -   A: It was possible to reach a pencil hardness of H with the        quantity of exposure being less than 300 mJ/cm².    -   B: It was possible to reach a pencil hardness of H with the        quantity of exposure being 300 mJ/cm² or more.

As shown in above Table 26, it will be recognized that when a basiccompound having a pKb value ranging from 3 to 7 is added to an inkjetink, it is possible to obtain an inkjet ink which is excellent in notonly shelf life but also in photosensitivity.

As explained above, according to one aspect of the present invention, itis possible to provide an inkjet ink without necessitating not only theemployment of an organic solvent but also an exposure system of largescale for obtaining printed matters of high quality.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An inkjet ink comprising: a photo-acid generating agent which iscapable of generating an acid as it is irradiated with-light; a colorcomponent; and an acid-polymerizable compound which can be polymerizedin the presence of an acid; wherein the acid-polymerizable solventcomprises a compound represented by the following general formula (1):R¹³—R¹⁴—(R¹³)_(p)   (1) (Wherein R¹³(s) is a group selected from thegroup consisting of a vinyl ether group, a group having a vinyl etherskeleton, an alkoxy group, substituted hydroxyl group and hydroxyl groupwherein at least one of R¹³(s) is vinyl ether group or a group having avinyl ether skeleton; R¹⁴ is a group comprising a substituted orunsubstituted cyclic skeleton and having a valence of (p+1); and p is apositive integer including zero).
 2. The inkjet ink according to claim1, wherein at least two of R¹³(s) in the compounds represented by thegeneral formula (1) are selected from vinyl ether group and a grouphaving a vinyl ether skeleton.
 3. The inkjet ink according to claim 1,wherein at least one of R¹³(s) in the compounds represented by thegeneral formula (1) is vinyl ether group which is directly linked to thecyclic skeleton.
 4. The inkjet ink according to claim 1, wherein thecyclic skeleton in the compounds represented by the general formula (1)is an alicyclic skeleton.
 5. The inkjet ink according to claim 4,wherein the cyclic skeleton is one represented by (VE1-a) or (VE1-b).

(Wherein X1 and Z1 are respectively alkylene group having 1 to 5 carbonatoms; Y1 is alkylene group having 1 to 2 carbon atoms; and k is 0 or1).
 6. The inkjet ink according to claim 5, wherein the compoundrepresented by the general formula (1) is selected from the compoundsrepresented by the following chemical formulas:


7. The inkjet ink according to claim 1, wherein the cyclic skeleton inthe compounds represented by the general formula (1) is a cyclichydrocarbon skeleton containing oxygen atom as a ring-constituting atom.8. The inkjet ink according to claim 7, wherein the cyclic skeleton isone represented by (VE2-a) or (VE2-b).

(Wherein at least one of X2, Y2 and Z1 contains at least one oxygenatom; X2 and Z2 are respectively alkylene group having 1 to 5 carbonatoms or bivalent organic group containing oxygen atom as ether linkage;Y2 is oxygen atom, alkylene group having 1 to 2 carbon atoms, orbivalent organic group containing oxygen atom as ether linkage; and k is0 or 1).
 9. The inkjet ink according to claim 8, wherein the compoundrepresented by the general formula (1) is selected from the compoundsrepresented by the following chemical formulas:


10. The inkjet ink according to claim 1, wherein the cyclic skeleton inthe compounds represented by the general formula (1) is an aromatic ringskeleton.
 11. The inkjet ink according to claim 1, wherein theacid-polymerizable compound further includes at least one compoundselected from cyclic carbonate and cyclic lactone compound.
 12. Theinkjet ink according to claim 1, wherein the acid-polymerizable compoundfurther includes an oxetane compound.
 13. The inkjet ink according toclaim 1, wherein the acid-polymerizable compound further includesdiglycidyl ether of neopentylglycol.
 14. The inkjet ink according toclaim 1, which further comprises at least one compound selected from thegroup consisting of a basic compound and a compound expressing basicityand exhibiting a pKb ranging from 3 to 7 at a temperature of 25° C. 15.An inkjet ink comprising: a photo-acid generating agent which is capableof generating an acid as it is irradiated with light; a color component;and an acid-polymerizable compound which can be polymerized in thepresence of an acid; wherein at least 40% of the acid-polymerizablesolvent is a compound represented by the following general formula (1),and the viscosity of said ink is not higher than 12 mPa.sec at theordinary temperature:R¹³—R¹⁴—(R¹³)_(p)   (1) (Wherein R¹³(s) is a group selected from thegroup consisting of a vinyl ether group, a group having a vinyl etherskeleton, an alkoxy group, substituted hydroxyl group and hydroxyl groupwherein at least one of R¹³(s) is vinyl ether group or a group having avinyl ether skeleton; R¹⁴ is a group comprising a substituted orunsubstituted cyclic skeleton and having a valence of (p+1); and p is apositive integer including zero).
 16. The inkjet ink according to claim15, which further comprises at least one compound selected from thegroup consisting of a basic compound and a compound expressing basicityand exhibiting a pKb ranging from 3 to 7 at a temperature of 25° C. 17.An inkjet ink comprising: a photo-acid generating agent which is capableof generating an acid as it is irradiated with light; a color component;and an acid-polymerizable compound which can be polymerized in thepresence of an acid; wherein at least 40% of the acid-polymerizablesolvent is a compound represented by the following general formula (1),and said ink further comprising an epoxy compound and/or oxetanecompound both having a solubility parameter of 19 MPa^(1/2) or more:R¹³—R¹⁴—(R¹³)_(p)   (1) (Wherein R¹³(s) is a group selected from thegroup consisting of a vinyl ether group, a group having a vinyl etherskeleton, an alkoxy group, substituted hydroxyl group and hydroxyl groupwherein at least one of R¹³(s) is vinyl ether group or a group having avinyl ether skeleton; R¹⁴ is a group comprising a substituted orunsubstituted cyclic skeleton and having a valence of (p+1); and p is apositive integer including zero).
 18. The inkjet ink according to claim17, which further comprises at least one compound selected from thegroup consisting of a basic compound and a compound expressing basicityand exhibiting a pKb ranging from 3 to 7 at a temperature of 25° C. 19.An inkjet ink comprising: a photo-acid generating agent which is capableof generating an acid as it is irradiated with light; a color component;and an acid-polymerizable compound which can be polymerized in thepresence of an acid; wherein at least 40% of the acid-polymerizablesolvent is a compound represented by the following general formula (1),and said ink further comprising an epoxy compound and/or oxetanecompound both having a viscosity exceeding 500 mPa.sec at the ordinarytemperature:R¹³—R¹⁴—(R¹³)_(p)   (1) (Wherein R¹³(s) is a group selected from thegroup consisting of a vinyl ether group, a group having a vinyl etherskeleton, an alkoxy group, substituted hydroxyl group and hydroxyl groupwherein at least one of R¹³(s) is vinyl ether group or a group having avinyl ether skeleton; R¹⁴ is a group comprising a substituted orunsubstituted cyclic skeleton and having a valence of (p+1); and p is apositive integer including zero).
 20. The inkjet ink according to claim19, which further comprises at least one compound selected from thegroup consisting of a basic compound and a compound expressing basicityand exhibiting a pKb ranging from 3 to 7 at a temperature of 25° C.